So, overall, the three linguistic branches of study are ultimately involved in basic word learning: phonology (sound), morphology (meaning) and the syntax (class). (See §0.3 for the role of syntax in word learning). Much feature theory is concerned with the “morphology” aspect of grammar; however, as we will see later, features can overflow or leak from one word to another, thereby affecting the overall syntax of a sentence. It is therefore appropriate not only to think about the specific characteristics of a word (in itself), but also about how these characteristics contribute to the overall composition of the sentence. In this sense, we will talk about specific lexical characteristics (at the level of the word itself), as well as how these characteristics acquire morphological properties that can affect other neighboring words in such a way as to cause syntactic construction (assembling words to form expressions, clauses and sentences). In a sense, the most fundamental level of morphology is actually the word – in the sense that morphology is defined as the smallest (free) unit of meaning. Obviously, the "word" constitutes the smallest unit of meaning, as opposed to the morphological (related) affixes -ing (progressive), -ed (past), etc. who (i) cannot be alone, (ii). ) have no real bearing on the meaning and (iii) only serve to a certain extent depending on the grammar. What makes the “word” so recognizable is the substantial nature to which it relates. This relationship is generally called the one-to-one relationship between sound and meaning (or concept). For example, the sound /tri/ is equivalent to the concept of a tree as it would be conceptualized in the speaker/hearer's mind. Then, the “word” can be defined as a morphological unit that contains a certain amount of meaning that can be conceptualized: tree/bush, car/bike, book/paper, walk/run, sleep/wake.,