One tool I use a lot for language learning and teaching is Google. It is a quick way of checking many things and is one of the most useful tools available, although it is not designed as a language tool, so its results should be treated with care. You could use any search engine, but Google is the one I use.
Why is language learning still one of the areas that lags behind in terms of the application of IT?
Most coursebooks teach mixed conditionals, but few look at just how mixed the conditionals can be.