Word can refer to the smallest unit that can be pronounced alone (contrasted with a morpheme, which is the smallest unit of meaning). A word may consist of a single morpheme or multiple morphemes. Words can be made of roots, affixes, and compounds. Words are combined to form larger linguistic elements like sentences, phrases, and clauses. Word classes are either open, accepting new additions, or closed, rarely accepting new items. A lexeme refers to a word and all its forms, while a morpheme is the smallest unit of meaning a word can be broken into. The mental lexicon contains a person's representations of words, differing from the lexicon as a general collection.

2. What is word?
 In language, a word is the smallest element that
may be pronounced in isolation. This contrasts
with a morpheme, which is the smallest unit of
meaning but will not necessarily stand on its own.
 A word may consist of a single morpheme.
For example:
Rock - Red - Quick - Run - Go
 Or several morpheme.
Rocks - Redness - Quickly - Running - Goes

3.  A complex word will typically include a root and one or more affixes.
Rock-s Run-ning
Red-ness Go-es
Quick-ly
 Or more than one root in a compound.
Black-board Down-fall
 Words can be put together to build larger elements of language.
Sentence
He threw a rock near to the
lake.
Phrase
A red rock
Clause
I threw a rock

4. Lexical word vs. grammatical word
Word classes may be either open or closed. An open class is one that
commonly accepts the addition of new words, while a closed class is
one to which new items are very rarely added. Open classes normally
contain large numbers of words, while closed classes are much smaller.
Typical open classes found in English and many other languages
are nouns, verbs (excluding auxiliary verbs, if these are regarded as a
separate class), adjectives, adverbs and interjections are often an open
class, though less familiar to English speakers, and are often open
to nonce words. Typical closed classes are prepositions (or
postpositions), determiners, conjunctions, and pronouns.

5.  A lexeme is an abstract unit of morphological
analysis in linguistics, that roughly corresponds to
a set of forms taken by a single word.
What is a Lexeme?
For example:
Run, runs, ran and running are forms of the same
lexeme, conventionally written as RUN
Find, finds, found, and finding are forms of the English
lexeme FIND

6. morphology
inflection derivation

7. Difference between Lexeme and Morpheme
Lexeme refers to a single word and all of its forms. For
example the word "go" in English has the forms "go" "goes"
"went" and "going". All of these words are from the same
lexeme "go.“
Morpheme refers to the smallest unit of meaning of a
word can be broken down into. For example the word
"cats" This can be broken down into "cat-s" "cat" carries the
meaning of the furry four legged animal and "-s" carries the
meaning of plural.

8. Lexicon v/s Mental lexicon
 Lexicon refers to the words used in a language or
by a person or group of people.
 The mental lexicon is defined as
a mental dictionary that contains information
regarding a word's meaning, pronunciation,
syntactic characteristics, and so on. The mental
lexicon is a construct used in linguistics and
psycholinguistics to refer to individual speakers
'lexical, or word, representations.

10. The mental lexicon differs from
the lexicon because it is not just a general
collection of words; instead, it deals with how
those words are activated, stored, processed,
and retrieved by each speaker. An individual’s
mental lexicon changes and grows as new
words are learned and is always developing.