Why what you learned isn't always what you hear.

When learning any new language, there can be some frustration when you try to listen to and understand native speakers. There are a few reasons for this.
The first and biggest reason is lazy pronunciation and slang. Native speakers of a language tend to be much less precise in the way they say and even use words often without noticing that they are doing it.
Have a look at this video of a British man talking about just this with his dialect of English.  

This happens with Gaelic too. Just think of all the times 'agus' is reduced to 's. 

This also brings us to the second main reason words may not sound they way you learned them: regional dialects. Gaelic has lots of them. Some times they say the same word differently from the way you learned, sometimes they may even use a different word. 

Here is a great set of maps to help show some of these dialectical differences

Scottish Gaelic Dialect Survey

Another good way of getting a handle on dialect differences is to hear them. Listen to BBC Radio Nan Gaidheal, look at some of the videos on e-Stòras  and Guthan nan Eilean check out some you tube videos ( try putting Nova Scotia Gaelic in the search box) and keep looking around on-line as there is new stuff all the time.