How do you know which form of a conjunction is correct?

Question

I'm learning Italian as a beginner and I'm struggling to understand when to use:

"o" vs "oppure"

"bensi" vs "ma"

"e" vs "ed"

What are the grammatical rules that dictate which is the correct form of or, but, or and to use in a sentence?

Answer 1

o / oppure

They are basically interchangeable.

"Oppure" can carry a slightly stronger value, highlighting an option that excludes all others: "non so se verrà nel mattino o nel pomeriggio, oppure se non verrà affatto" ("I do not know whether he/she will come in the morning or in the afternoon or if he/she will not come at all").

"Oppure" tends also to be preferred before words beginning with "o", for euphonic reason: "Non so decidere se preferisco Amleto oppure Otello" ("I cannot decide whether I like more Hamlet or Othello").

bensì / ma

As a rule of thumb, you can always replace "bensì" with "ma", but you cannot always replace "ma" with "bensì"

"Bensì" indicates that two options are mutually exclusive: "mi ha convinto non con le parole bensì/ma con l'esempio*" ("He/She convinced me not with his/her words but with his/her example").

On the other hand, "ma" can be used also to introduce a sentence contrasting with some given premises: "Odio i boschi ma verrò con te per farti compagnia" ("I hate woods but I will come with you to keep you company"). "Bensì" cannot be used in this case.

e / ed

They are exactly the same: you can use "ed" in front of words beginning with a vowels, if it sounds better to you: "Ci siamo visti ieri e/ed oggi" ("We met yesterday and today").

Anyway you never use "ed" in front of words beginning with a consonant; in that case you always use "e"

[EDIT] As DaG rightly mentioned in the comments, nowadays the trend is to use "ed" only in front of words beginning with "e" rather than with any vowel, with the notable exception of word beginning with "ed", where the use of "ed" could cause a cacophonic repetition: "è gentile e educato ("he is kind and well-mannered").