Is it a noun, part of a verb (is the verb an auxiliary or a main verb?), adjective, verb, adverb, pronoun, determiner, conjunction, preposition, exclamation?
Some of these words never change but some do!!! It's important to know. The way you change them depends how you want to use them. e.g. noun=sleep, verb=sleep, adjective=sleepy, adverb=sleepily.
'It' is a pronoun. 'It' replaces a noun which is singular and not male (he) or female (she).
'They' can be anything. (see section on pronouns)
Conjunctions, prepositions, and exclamations don't change.
Also it's important to know what word follows what word e.g. 'she hardly works' v. 'she works hard'.
Which is right?
They both are. They mean something completely different. Look up the meanings in your dictionary; you will find that 'work hard' means 'work a lot' but we don't say that in English.
There are groups of words which have a meaning which is beyond word classes too e.g. 'I look forward to hearing from you' means 'please answer me as soon as possible' in formal English.
Try to put each word in a 'word class':
e.g. I work hard. (Pronoun, verb (Present Simple), irregular adverb which doesn't change its form from the adjective 'hard')
2. She loves you, yeah! yeah!
3. Books are my best friends.
4. The houses between me and the river almost cut off the view from my window.
5. There is a queue of cars going over the bridge.
6. The weather is awful, isn't it?
7. I'll find out who did this.
8. She's got her mother's hair and her father's eyes because she takes after both her parents.
9. Life begins to slow down in the autumn.
10. The fallen leaves will make a protective shelter.
2. Pronoun, verb, pronoun, exclamation.
3. Plural Noun, verb 'be', possessive determiner (my), superlative form of the adjective 'good' (best), noun.
4. Determiner (the)+ noun, preposition (between), personal pronoun (object) (us), conjunction (and) for joining two ideas together; determiner + noun, adverb, two word verb (cut off), determiner + noun, preposition, possessive determiner, noun.
5. Adverb, verb 'be', indefinite article + noun, present participle, preposition, determiner + noun.
6. Determiner + noun, verb 'be', adjective, negative auxiliary + pronoun (matching the subject, the weather) - this is called a tag question.
7. Pronoun, auxiliary 'will' contraction 'll', two word verb (find out), relative pronoun (who) subject of the following Simple Past tense verb (do), demonstrative pronoun (this).
8.Pronoun, contraction of auxiliary 'has' (verb 'have'), two word verb 'have got' means the same as 'have', possessive determiner + noun + possessive 's', noun, conjunction, possessive determiner + noun + possessive 's', noun, conjunction, pronoun, two word verb ('take after' - a 'phrasal verb' because the two words together change their meaning when put together, possessive determiner + noun.
9. Abstract noun (needs no determiner), verb + to + two word verb (slow down), 'adverbial phrase' = preposition + determiner + noun.
10. Determiner, adjective derived from past participle of verb (to fall), irregular plural noun (leaf - 2 leaves), auxiliary (will) + verb form (make), indefinite article + adjective + noun.
n.b. you need a determiner in front of a singular noun unless it's an abstract noun (see 9 above).
See The A - Z of English Grammar & Usage.