Grammar detail: difference between だけ and しか

Most importantly, しか is followed by a negative verb, whereas だけ is usually followed by a positive verb.

However, there is a more subtle difference: だけ contains no emotional implications - it merely expresses a fact. しか, on the other hand, implies an underlying (negative) judgement. Using the above examples, the first simply states the fact - you only understand English. The second, however, implies perhaps a tone of apology, or pessimism, or perhaps the idea that you think you really ought to understand another language.