1. Phoneme: The smallest unit of sound that can distinguish meaning. For example, the words "cat" and "hat" differ by only one phoneme, /k/ and /h/.
2. Morpheme: The smallest unit of meaning in a language. It can be a word, like "cat," or a part of a word, like the plural morpheme "-s" in "cats."
3. Word: A unit of meaning that can stand alone. It is composed of one or more morphemes.
4. Phrase: A group of words that functions as a unit and does not contain a subject and verb. For example, "the big red ball" is a noun phrase.
5. Clause: A group of words that contains a subject and a verb. It can be independent (a complete sentence) or dependent (part of a larger sentence). For example, "The dog barked" is an independent clause, while "because the dog barked" is a dependent clause.
These units build upon each other in a hierarchical structure, forming the foundation of English grammar.