I would suggest thinking beyond the issue of grammar. Let me explain. Grammar is just a structure for how words operate. Thinking on this level about formulas and sentence constructions can be very boring, and you won't tend to remember things that are boring.
Instead, try reading. A lot. Anything you're interested in. Find topics you care about and read multiple articles, websites, books, anything. Just enjoy learning what excites you. Then, listen to cultivated and smart people talking about subjects that interest you. Listen to TED talks on Youtube, for example. If you find one you really like, listen to it many times. You will absorb the ideas together with the manner in which these ideas are expressed. The grammar will sink into your mind in the most natural way, as you gradually memorize the ideas and the structure of the words at the same time. In other words, you'll begin to think in the language you want to learn.
English grammar is really not that difficult. If you really internalize some written and spoken text, the patterns reveal themselves pretty clearly and you will notice them.
Finally, write your own thoughts and have an educated person (such as an English-speaking professor) edit your writing for you. Many people will be willing to do this for someone who is serious about learning. If you have an open and willing mind for learning (instead of feeling embarrassed or defensive when receiving corrections), you will improve noticeably every time you do this. Most people, including university students, don't want to be told what's wrong with their writing. If you are willing to learn though, you can develop a level of sophistication beyond that of many native speakers.
Final thought: even the most seasoned writers, thinkers, and speakers continue to do these things at all times. If your motivation is sincere, you will always enjoy learning and improving. This is the greatest reward, in my view.
I'm afraid there is no shortcut to learning grammar PERMANENTLY. Yeah, you can always go ahead and read some quick tutorials or cram a bunch of rules, but it won't be mapped to your long term memory (unless of course, you have an eidetic memory).
There are however a lot of fun ways to improve your grammar slowly and effectively. Choose your pick :
- Read novels. It doesn't matter whether you read classic literature or modern sci-fi. Just try and explore a wide collection of novels. Some will be written in the first person, some in the third person. All these angles will help you enlarge your database of Grammar. My personal favorite is Harry Potter.
- You can also begin by watching English TV series. It's another way of enriching your vocabulary and grammar as well as have an amazing time(if you choose your tv series wisely).
- Try and communicate with yourself in English. It doesn't need to be out loud, just make sure your brain is forever engaged in converting sentences into English. Whenever you're stuck, you can ask someone for help or simply Google it.
- Newspaper. It never really worked out for me as I can hardly get through one news article in a day, but it might do wonders for you.
Firstly, try remembering words in groups. Like, introvert, extrovert, ambivert. Again, ingenious, ingenious, indigenous, etc.
Secondly, find the roots of the words! Like 'Greg', 'bene', 'mala', 'flu', 'con', etc. Now see which particular words have these words mentioned in them and the likes. Greg means crowd, bene is something good, mala is something bad, etc. From Greg, you get gregarious, etc. From bene, you get benevolent, benefit, beneficial, etc. From flu, you get fluent, effluvium, affluent, confluence, mellifluous, etc. So when you come across words that have these small words in them, you can easily remember what it means. This way, remembering words becomes much easier.
Thirdly, make sentences with the words. Example: Epicure. The word means "one who has a love for very fine things." Now, if you have difficulty in remembering the word, make a sentence with it. Like, a man with epicurean tendencies must have a fat wallet. That way, you can remember the word and the meaning.
Fourthly, picture them. If you picture or visualize the word or the sentence that you have made in your mind, remembering the word becomes very very easy, trust me. Try this one.
Fifthly, connect each and every word with the eight parts of speech. If the given word is a noun, convert it into an adjective and vice versa. Doing this, you will be learning two words at the same time meaning the same thing.
Simply follow Wren & Martin for grammar. It will help a lot. Basic plus standard level.
Just write and practice all the above things that I have mentioned. You can also take up dictation. Ask someone to dictate a particular passage to you. Make sure their pronunciation is correct.
Follow these sincerely and you will master in English.