Hey Matvey, how are you? I hope you're doing just fine!
I shall provide you with an explanation of the usage of these words. I should say that I don't believe that there is quite a difference between them: only a ''slight one''. So, you must know this:
* ''Often'' and ''Frequently'' are used interchangeably in most cases, but ''often'' implies numerous repetitions, like: ''We often go there''; whereas ''frequently'' suggests repetition at comparatively short intervals, for example: ''It happens frequently.''
Here are some more examples for you:
1) ''I was often hungry''
2) ''They had often been arguing with each other''
3) ''Iron supplements are frequently given to pregnant women''
4) ''I frequently drop by my grandma's house to make her a visit''
5) ''My brother often gets into trouble when I'm not around''
Hope that helps!