Here's a quickie: you should probably use "learned" instead of "learnt," at least when corresponding with native English speakers.
In the US and Canada, "learnt" is almost never used (once in every 500 usages). I suspect that the vast majority of the times I've typed the word "learnt" is right here in this post. We just don't use it in North America.
In British (and, presumably, Australian, New Zealand, and South African too) writing, it's considerably more common. However, even there, it's 1 in every 3 usages. (See: Learned vs. learnt
Either way, if you're corresponding with a native English speaker, "learned" is going to be the more common form.
I'm not saying that "learnt" is incorrect. On the contrary (in certain usages), "learnt" is perfectly correct and you will not be doing anything grammatically wrong in using it.
However, keep in mind that many Americans will see this as a sign that you aren't very comfortable in English. Are they wrong for doing so? Yes. They should
be educated that "learnt" is correct and is commonly used (though still probably less so than "learned") in the UK.
However, it's probably easier for you to adjust your speech than to adjust the attitudes of about 300 million people.That
is why I'm saying that it's best to just use "learned."
- It's simple. "Learned" can be used anywhere that "learnt" can be used.
- It'll be much more familiar with Americans and make them less likely to (unfairly) judge you as someone not comfortable in English.
- With people from the UK, "learned" will be equally or more familiar than "learnt."
So while you won't be doing anything grammatically incorrect in using "learnt," it might not be in your best interest. If you still wish to use "learnt" though, because you really, really want to use this as an opportunity to educate Americans, then go right ahead.