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One thing (amongst many) that makes Gaelic simpler than some other European languages is that it has no indefinite article ("a") for nouns, just the definite article ("the"), although sadly gender matters, so you have, for standalone nouns:

• an t-eilean (m) (the island)
  
• a' bhliadhna (f) (the year)
• am fraoch (m) (the heather)
• an leabhar (m) (the book)
• a' chraobh (f) (the tree)

When you want to say "in a", then it all goes swimmingly:

• am bheinn ann an t-eilean
• an dealbh ann an leabhar
• an ubhal ann an craobh

It's when you want to say "in the" that the trouble starts, because suddenly the noun modifies:

• am bheinn anns an eilean
• an là anns a' bhliadhna
• na dealbhan anns an leabhar
• an ubhal anns a' chraobh
• am madadh-ruadh anns an fhraoich
  

Sometimes the change in the noun is quite drastic, as in "oifis a' Phuist" (from "Am Post") and "anns an fhraoich" above - not only have you to lenite, you have to shove an "i" in as well if you can. (I think this is the genitive case, but I wouldn't swear to it, although it is similar to the word change when addressing a male, eg Seumas -> a Sheumais.) I came across this in unit 3/4 of An Cùrsa Inntrigidh, and even now, at unit 11, I still haven't quite got the hang of it. I'm now learning that this applies to anything that implies "of the", eg "bileag a' bhotail" (bottle label).

There are grammatical rules, but there's no way you can think fast enough to apply them in normal conversation, so you just have to internalise them through repetition or by keeping standard examples in mind. I quite like remembering countries:

• anns a' Bheilg
• anns an Fhraing
• anns an t-Suain
• anns a' Ghearmailt

I suppose the simple guideline is one that appears to work for much of Gaelic - if in doubt, lenite ;-)

More info on articles in the Wikipedia: Scots Gaelic entry. See also Lessons 3 and 6 at TAIC.