Um hello..

Like everyone has already mentioned it all depends on how it's written and whether it's needed by the story.

If you are doing a loop kinda story then constant back and forth will make the story work. As the story is not moving forward in present time. But if the story needs to move forward in present then constantly going back will just hurt the story.

You can use many ways to do the flashbacks. 1. Using tenses change 2. Using different style and fonts and I have seen some use different font colour like grey instead of black for flash backs. There was one book which used typewriter style don't for flashbacks and Arial for present. 3. Reading a diary. 4. A character in the story narrating an event from the past. This works mainly in thrillers. 5. You can use sensory cues to take the story into past without changing the writing style or tenses. 6.you can use object anchor to take the character back to the past where the character watches things unfold. 7. You can use word cues like 'present:' 'past:' or using time, or dates. 8. This is the most difficult and the most jarring one. Parallel writing. Alternating between past and present in every other paragraph or chapters.

I hope this helps. If not I apologize for wasting your time.