If I do not understand the new subject during the lesson, I will ask my teacher. (not understand-ask)

Answer:

If I do not understand the new subject during the lesson, I will ask my teacher.

This is a first conditional sentence. The first conditional is used to talk about things that are possible in the future. The structure is: If + present simple, will + base form.

Это предложение с первым условием. Первое условие используется для разговора о вещах, которые возможны в будущем. Структура такова: If + present simple, will + базовая форма.