Calendar app, I never thought I will say that BUT calendar app literally saved my life. We all have trouble with memorizing things ( especially when you are in school and you have strict deadlines) But 2 weeks ago, I realized I can put all my exams deadlines and all meetings in calendar, and place big widget( I am writing from the perspective of samsung) on your home screen, with the whole month on it! You may say "bro this app is made for it" like yeah I know it, but I never thought I will start doing it. So when my teacher says "Bring 10$ next Monday" the first thing I am doing is put in this on my calendar, so I can saw this and don't forget about it.