My humble respect to you. I'm a student and I study a lot. Would you give me, please, some advice on how to enhance my concentration and ability to memorize?
If you are like most young men whom I know, you have a strong ability to concentrate. Not all the time, but if there is something that is particularly interesting to you, it would take someone knocking you over before you notice your surroundings again. Similarly, I've seen young men who claim to have a poor memory for school work, yet who can quote me sports statistics for years back in small detail.
Thus the problem is rarely ability, but a lack of sufficient interest in a particular subject. I would expect that I would not be at my best on a subject that isn't my favorite topic, but even then, I know that ignoring subjects that I don't like is dangerous. If I alone picked what I know and don't know, then I would become a fool. "How long, you simple ones, will you love simplicity? For scorners delight in their scorning, and fools hate knowledge" (Proverbs 1:22). It is easy to mock what you don't know, but it doesn't make you a better person.
Learning is easier when you understand why things are as they are. You can memorize all the grammar rules you want (and in English there are tons of rules with all sorts of exceptions), or you can sit down and figure out why those rules exist. Knowing the logic behind a subject makes learning the facts easier. "A scoffer seeks wisdom and does not find it, but knowledge is easy to him who understands" (Proverbs 14:6). So instead of tossing out a subject because it looks silly to you, seek instead to figure out how it works and why it works.
A part of learning is also accepting that you don't know everything, so you are going to get things wrong. Some people can't stand to be wrong. They hate being corrected. "Whoever loves instruction loves knowledge, but he who hates correction is stupid" (Proverbs 12:1). In order to learn, I have to take the risk of being shown that I can be wrong.
Finally, nothing helps you learn a subject better than helping someone else learn. "The lips of the wise disperse knowledge, but the heart of the fool does not do so" (Proverbs 15:7). Figuring out how to explain something to another person helps you to see the logic of the subject, and makes it more relevant to you.