Change of tone with measure words

Hello everyone! I’m a very beginner learner of Mandarin. I took the “measure words” class today and I was reviewing the lesson and I realized I have an unanswered question. I remember that the world “1” is Yi with tone 1, but for some counting words such as “wan” (碗) and “ping” (瓶) the tone of “1/yi” changes. (see picture below). Is there a rule about when the tone of the number before the counter changes?

Thanks in advance!

Hi, there is a rule for this.

一 as a standalone character has the 1st tone, but it interacts with things that follow it.

If the following word has a 1st, 2nd or 3rd tone, then 一 takes a 4th tone.
If the following word has a 4th tone, then 一 takes a 2nd tone.

Hope this helps!

yes! this is helpful. thanks!!