Fòram na Gàidhlig

I've seen throughout 'Speaking Our Language' different words for 'if', i.e 'ma', 'nam'/'nan' and 'mura'. 'Nam' and 'nam' seem to be used a lot in the dialogues involving the conditional mood, but I was wondering what's the difference between these forms. Are they basically interchangeable, or can you only use certain ones for certain situations?

Ma is a "real" if -- it's a possible, likely thing... just that we don't know for certain yet. e.g. Ma thig tu a-nochd, ceannaichidh mi pinnt dhut (If you [will] come tonight, I'll buy you a drink.)

Nan (or nam before a labial consonant) is an unreal, hypothetical if, with a similar sense to the subjunctive in many European languages. Traditional it takes the conditional in both parts of the sentence, but in modern usage it often follows the patterns of English if-sentences. Used traditionally, Dhèanainns' e nam bidhinns' ann -- lit "I would do it if I would be there". As I understand it this can be any of ""I would do it if I was [going to be] there", "I would do it if I was there [now]" and "I would have done it if I had been there".

Mur/Mura is the negative form of both "ma" and "nan".
ma thig tu = if you come
mur tig tu = if you don't come
nam biodh i = if she were/if she had been
mura biodh i = if she weren't/if she hadn't been

edit: corrected as per Akerbeltz & Gunchleoc. I now wish I'd stuck with a regular verb...

Errr yes, what Niall said but be aware that the future relative of thig is thig, i.e. ma thig thu. *thigeas is not considered good Gaelic, at least not at the moment.

...and "mur tig thu". Let's use a regular verb:

ma chuireas
mur(a) cuir
nan cuireadh
mur(a) cuireadh