How do you give code review feedback as the junior without sounding like you're overstepping?

I'm about 8 months in and I'm getting added as a reviewer on PRs now, including from people way more senior than me. Half the time I spot something (a missed edge case, a confusing name) but I freeze on how to phrase it. 'Isn't this a bug?' feels presumptuous, staying silent feels useless.

For those further along: how did you learn to leave review comments that are actually helpful without being either a doormat or a know-it-all? Any phrasing that consistently lands well?