The best way to learn English grammar is the same way that you learned the grammar of your own language; by listening to English and reading it; by speaking it and writing it. But many learners enjoy doing grammar work and think that it helps them to learn English more quickly!
It is certainly true that doing grammar exercises like the ones on these web pages can help a little. For example, if you read and think about the explanations, they may lead you to understand better why a particular grammar form is used in a particular situation. The exercises can also be used to test yourself on what you already know (or think you do!)
You need to remember, however, that some people find they are very good at doing grammar work, but make lots of mistakes when they come to write themselves. (And it is even more of a problem when they come to speak.) Others don't do very well in grammar exercises but can write and speak accurately. So the best thing is not to spend too much time on grammar alone. You would do better to read an interesting book.
And never forget: no-one learns to speak a language without making grammar mistakes. The main thing is that you can communicate what you want to say. Look at how poor JM loses the chance to make a new friend.