I'm going to a type a few commands can you please correct them if neccessary? I've had a bit of trouble with these.
I've bolded the main parts so its easier for you

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lets just say "it" is a "el libro"
- to a good friend (informal) /
tú
Cómpralo - "buy it"
(drop the final "s" add the pronoun, ect. And is the accent in the right place??)
(Tú) no lo compres - "Dont buy it"
(in the negative informal the verb comprar takes the subjunctive form??)
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- to good friends (informal) /
vosotros
Cómpradlo - "buy it"
(take the infinitive and replace the final "r" with a "d" /again is the accent in the right spot, also is the pronoun in the right spot? It just sounded a little strange to have a pronoun after a "d" so I wasn't sure)
(Vosotros) no lo compreís - "dont buy it"
(uses subjunctive form??)
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As far as I could tell every thing in the imperative (command) mood uses the present subjunctive form of the very, except in the
Affirmative Tú
and
Affirmative Vosotros
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Muchos gracías para tu ayuda