1. "if i were" or "if i was"
If I were. Don't really remember the explanation for this, but I think that the conditional tense of the verb "to be" is, in all forms, "were".
If I were
If you were
If he were
etc
2. why can't i say furnitures?
For the same reason that you can't say "breads", or "moneys".
Moira has developed this point very well.
3. why is it "sleep in my bed" and not "sleep on my bed"?
Because the preposition "in" suggests more the idea of "above", "on top of" than the preposition "on" does. Well, it's just convention. The english prepositions are a tough cookie to eat, and that is the section of the language in which I'm least comfortable (
ex aequo with phrasal verbs)
4. why is it "In Rome" and not "at Rome"
Because "in" is the preposition used for cities and countries. This is a mere convention. In things related to time the same conventions exist:
"At"
nine o'clock
midnight
"On"
Monday
Tuesday
"In"
1927
14th century
As for the book, I'd recommend any grammar. They will surely explain this to you better then I did.
Hope I was of any help, and the other members of this forum will correct me if they see any fault in my explanations. Welcome aboard,
tridentseven!