The key principles associated with the difference between Morphology and Syntax

In this essay, I will first describe what the fields of morphology and syntax study in order to highlight issues that fall between the two categories. Then, I will intend to give a description of what problems may arise when teaching English to non-native speakers.
Morphology intends to study the internal structure of words. It can be said that words are the smallest units of syntax. For example, a proficient speaker of English can recognize that the words boy, boys, and boyhood are closely related. English speakers are able to recognize how these words are related from their unspoken knowledge of the rules of word-formation in English. They observe that boy is to boys what girl is to girls. Similarly, boy is to boyhood what neighbour is to neighbourhood. Speakers are able to recognise patterns in the way words are formed from smaller units.
Within morphology, we have a distinction between word and lexeme. That is, there are two senses of “word”. The first sense of “word” says that boy and boys are “the same word”, or the same lexeme. On the other hand, boy and boys are different word forms of the same lexeme.
Taking into account the notion of lexeme, we can distinguish two kinds of morphological rules. While a kind of morphological rules relate different forms of the same lexeme; other rules relate different lexemes. The first type of rules are inflectional rules, on the other hand, the second type of rules are derivational and compositional rules. The English past, e.g. walk – walked, is an example of an inflectional rule. In this case, we just obtain different forms of the same lexeme. Formation of new lexemes, on the other hand, is examples of derivational and compositional rules; e.g. neighbour – neighbourhood or dream – dreamcatcher.
On the other hand, syntax is the study of the rules of a language that show how the words of that language need to be assembled in order to make a grammatical sentence. In other words, syntax is the study of the architecture of phrases, clauses, and sentences.
A theory of syntax has to describe all the well-formed, acceptable sentences in any given language.
In order to form grammatical sentences we need to combine a series of words. Word categories are theoretical notions to explain the fact that not all words behave in the same way. We do not want to use vague notions such as “nouns are persons, places, or things” (the word run can be a noun or a verb), “verbs refer to actions” (destruction is an “action”, but is a noun), and “prepositions are words referring to locations” (Paris is a location, but is a noun). To determine different parts of speech or word categories, we need to observe phonological, morphological, and distributional evidence.
Morphologically, nouns can be pluralized (boys, women) and verbs cannot. Nouns and verbs can form complex words made up of more than one morpheme, but prepositions cannot; they never change. Distributionally, nouns occur in different parts of a sentence than verbs do. For example, nouns can be pre-modified by adjectives (very big boy, pretty woman, etc.) yet verbs cannot (*very big know); nouns can be quantified and specified (e.g., the boy, a boy), verbs cannot (*a / the know). So, a verb cannot be substituted for a noun, and vice versa.
Because of these phonological, morphological, and distributional facts, linguists have hypothesized a restricted set of lexical categories such as Nouns, Verbs, Adjectives, and Prepositions, as well as a set of functional categories such as Determiners (the, this, some, many, etc.), Complementizers (that, whether, for, etc.), and Inflections (for example, modals such as may and must, and tense and agreement morphemes).
Different parts of speech are not arranged in a one-level left-to-right serial order. Evidence suggests that they form phrasal categories, and lexical, functional, and phrasal categories are arranged in a hierarchical structure to form clauses and sentences.
There are cases where more than one structure can be assigned to a particular phrase. For example, she fed her piranha fish fingers has three different interpretations, or three different structures. First, this sentence can mean that she fed someone a dish of piranha fish fingers. Secondly, it can mean that she has some piranha fish to which she fed fish fingers. Lastly, it can mean that she has piranha fish and she fed them fingers.
In general, in syntax we argue that every phrasal category contains a head; and that the head and its phrasal counterparts share the same properties. So, for instance, an NP must contain an N, which is the head of the NP, a VP must contain a V, etc. Moreover, the head and its phrase share properties. For instance, if a head noun is plural, so is the entire NP; e.g., The girls are wild.
An analysis of English sentences tells us that a sentence consists of a noun phrase and a predicate (i.e., a verb phrase). An NP consists of a noun (compulsory), which may be preceded by a determiner. A verb phrase consists of a verb (e.g. slept), and potentially other optional elements, including another NP (e.g. fixed the car), a PP (e.g. sent a card to his girlfriend), or even another sentence (e.g. thought that was a better idea). A prepositional phrase may include a preposition followed by an NP, e.g. in the house.
In linguistics, we also study the field of morphosyntax, describes grammatical properties that fall both in the fields of syntax and morphology.
Lexemes may have the following inflectional categories: tense, aspect, mood, number, gender or case. In English, different moods are achieved using different modal verbs. However, this has the syntactic requirement that the main lexical verb has to follow the modal and it has to appear in its infinitive form (despite of the number and person of the subject). Modal auxiliaries do not have inflections. So, sentence (1) is grammatical, but sentence (2) and (3) are not:

Aspect has similar requirements as it is marked by an auxiliary, have for perfective and be for progressive. In perfective aspect, then we need to make use of the past participle form of the lexical verb; and in progressive aspect, we need to have the verb in its gerund or present participle form:

In contrast with modals, the auxiliaries have and be need to agree with the subject in person and number.
Tense, gender, number, and case are all indicated with different morphological endings depending on their function inside the sentence. Past tense in regular verbs is formed by adding -ed. Irregulars has special forms. However, future tense is formed with the aid of an auxiliary, namely will.
Not all nouns in English are marked for case and gender. Only personal pronouns in English can be organized using the categories of person (1st, 2nd, 3rd.), number (singular vs. plural), gender (masculine, feminine, neuter), and case (subjective, objective, and possessive). We then need to outline to the learner of English that if a personal pronoun appears in different parts of the sentence, a different inflection of that pronoun must be used to make a grammatical sentence. For instance, we need to outline that a second person plural pronoun when appearing as a subject needs to be the form we as in We won the trophy. But if that same pronoun is the object of the sentence, then we need to use the form us as in They gave us the trophy. And if that pronoun needs to express possession, then we will need to use the form our if followed by a noun or ours if not: this is our trophy vs. this is ours. In addition, if a pronoun is used to indicate a reflexive form, then ourselves needs to be used as in We won the trophy ourselves.
The inflectional categories used to classify word-forms cannot be chosen at random; they must be categories that are related to the syntactic rules of the language. For example, person and number are categories that can be used to define paradigms in English since English has grammatical agreement rules that expect the verb in a sentence to occur in an inflectional form that corresponds to the person and number of the subject. We also need to let the learner know that any NP in subject position needs to agree with the verb. However, since English does not have a very complex inflectional morphology, that will only affect the third person singular and irregular verbs such as be.
The syntactic rules of English are concerned about the difference between boy and boys, because the choice between these two word forms establishes which form of the verb has to be used. In contrast, no syntactic rule of English is concerned about the difference between dream and dreamcatcher, or boy and boyhood. These are all examples of nouns, and they generally behave like any other noun.
An important difference between inflection and word-formation is that inflected word-forms of lexemes are organized into paradigms, and in turn, these are defined by the requirements of syntactic rules. On the other hand, the rules of derivation and composition are not restricted by any equivalent requirements of syntax. Inflection is relevant to syntax, and derivation and composition are not.
For an English learner with a language without a rich inflectional morphology, it will be quite difficult to understand that words require of different word endings depending on their position and role in the sentence. However, for those who are learning English whose first language is a highly synthetic language (that is, it relies on adpositions, affixes, and internal modifications of lexemes to establish relations and roles within the sentence), it will be difficult to come to terms with the fact that English is an analytic language (that is, it relies heavily on word order or prepositions to indicate relations in the sentence). And a further puzzlement will come after being told that in some occasions either a clitic or a preposition can be used for the same thing. That is, a genitive’s can be used as well as an of structure to indicate possession. This is of course a result from the fact that Old English was a synthetic language and Modern English is an analytic language, there are still some of the features of Old English remaining (it also applies to personal pronouns).