Grammar fix

I'm not following

say you're lost

A polite way to admit you can't keep up with or understand an explanation, so the speaker slows down or explains again.

expressionB1common

Hear it in context

Notice where the phrase sounds natural.

Sorry, I'm not following — could you go over that last step again?

A: So you take the derivative, then substitute back.
B: Hang on, I'm not following — which bit do I substitute?

One small grammar move makes it sound right.

Not: Sorry, I'm not ~~follow.~~

Say: Sorry, I'm not following.

Use the continuous 'I'm not following', not 'I'm not follow'.