This is an overview of Spanish conjunctions. First all of them are listed alphabetically for reference, followed by more detailed explanation and rules.
All Spanish conjunctions
aunque = although, though
como = like, as
cuando = when
donde = where
e = and (before i- or hi-)
entonces = then
mas = but (more formal)
mientras = while, whereas, as long as
ni = neither, nor, not even
o = or
pero = but
porque = because
pues = then
que = that, which
si = if, whether
sino = rather
u = or (before o- or ho-)
y = and
And in Spanish: y or e?
The conjunction and in Spanish is y in most cases.
Hablo inglés y español.
I speak English and Spanish.
However, in some cases y changes to e. This is before words that start with i- or hi-, as those are pronounced quite like y, and therefore using the word e instead of y makes pronunciation easier.
Hablo español e inglés.
I speak Spanish and English.
Español e inglés is much easier to pronounce than español y inglés would be.
There is one exception to this exception. Before words that start with hie- we still use y:
agua y hielo = water and ice
To sum up, and in Spanish is:
e before words that start with i- or hi- (but not hie-)
y before words that start with hie- and anything else
Or in Spanish: o or u?
Or is o in most cases. Just like y changes to e, o changes to u when placed before some words for the same reason – easier pronunciation.
Before words that start with o- or ho-, use u as or:
verano u otoño = summer or autumn
In all other cases, use o:
verano o invierno = summer or winter
But in Spanish: pero, mas, or sino?
There are three conjunctions in Spanish which can be used like but in English. Two of them (pero and mas) are more or less interchangable, while the third (sino) is not.
The most common word for Spanish but is pero.
No soy español, pero hablo un poco de español.
I am not Spanish, but I speak a little of Spanish.
Most native speakers consider the conjunction mas an equivalent to pero, possible to be used in the same situations. Mas is more formal, a bit old style feeling, and used less frequently in everyday Spanish than pero. Just make sure you don't put an accent mark over the a in mas; otherwise it would mean something completely different (más = more).
Sino, on the other hand, is not interchangeable with pero or mas. It is used as but to express the contrast in situations when something is not true, but something else is. You say both things, one is true and the other is not, and you put the word sino between them.
No hablo italiano, sino español.
I don't speak Italian, but (I do speak) Spanish (instead).
Neither... nor... = ni ... ni ...
When none of the two things are true (neither ... nor ... in English), put the conjunction ni before each:
No hablo ni italiano ni español.
I speak neither Italian nor Spanish.
Notice that in Spanish you say "no hablo ni ..." (double negative), while in English you say "I speak neither ..." (only one negative).