In spite of my preference for well-spoken English, I will admit that there are times I lapse into Singlish for a number of reasons. It brings me comfort, it's fun to listen to every once in a while, and most importantly, it's incredibly effective when you need to get your meaning across in a minimal number of words.

For instance, instead of saying, "what should we do now?" after one has listed a bunch of options, one can opt to use "so how?" Another more recent example is "whatever lah!" in place "it really doesn't matter what you choose as it makes no difference."

My Indonesian colleague has been incredibly tickled by the expediency and efficiency of using Singlish to communicate with my fellow Singaporeans. The only problem in attempting to educate her in the proper usage of Singlish - and a problem frequently faced by other non-locals, I might add - is how to explain the proper placement of "lah", "leh", "lor" and "ah" and how to pronounce the non-English words properly. For some reason, foreigners, when attempting to show bond with us, tend to add "lah" to the end of every sentence, and use "wah lau" (an expression similar to how "oh my god" is usually used) in response to something we say.

The thing is, there are, believe it or not, instances when a "lah" at the end of a sentence sounds terribly out of place. It is one of the most common words heard out of a typical Singaporean's mouth, I'll admit. We say things like "okay lah", "no lah" and "you know lah" very often. But the placement of "leh" and "lah" can alter the tone and sometimes even the meaning of the phrase. Adding a "lah" and a "leh" to "don't like that" can alter the tone. In both cases, the meaning is the same: "Don't be like that." However, "don't like that lah" sounds more playful and casual to me, whereas "don't like that leh" sounds slightly more pleading and apologetic.

As for "ah", the only instance that crosses my mind right now is its use in "really ah?", which means, well, "really?" And the response to that, in my case, is usually "okay lor" which means "okay". Note here that "okay lah" doesn't quite go as a response to the "really ah?" It's hard to explain but to those well-versed in the usage of Singlish, it comes naturally. The sound isn't right. It just isn't.

Links:
thedean's faculty on how much time could be saved if we all spoke Singlish.
mr brown on the meaning of lah. (42! 42! Oh... the meaning of lah. Pai seh. My bad.)