Who knows English well? I want to brush up on my grammar. Especially the tenses - it's just awful... Maybe someone knows good sites or YouTube channels where this is explained simply and clearly?

Don't listen to anyone; you can learn English and speak fluently and competently. If you don't have enough time, try downloading this promova english app. It works on both Android and iOS, which is very convenient. You'll never miss a class because the app will remind you about it. Very well-made app, filled with the most important and relevant lectures and the most effective exercises. Highly recommended!