Learn Farsi: مادر انگورها را شست.

The word را is a marker that shows the direct object of the sentence is specific. Because the English translation says "the grapes" (meaning specific grapes, not just any grapes), we must use را right after the object to show exactly what the mother washed.

What happens if I leave out را and just say مادر انگورها شست?

Leaving out را here makes the sentence sound unnatural. Because انگورها has the plural marker ها, it usually implies a specific group of grapes in this context. If you wanted to say "Mother washed grapes" in a general, non-specific way, you would drop both the plural marker and را, saying مادر انگور شست.

Why isn't there a word for "the" before مادر or انگورها?

Farsi doesn't have a direct equivalent for the English definite article "the". For the subject مادر, being at the beginning of the sentence usually makes it understood as specific. For the object انگورها, the marker را does the job of telling us that we are talking about "the" grapes.

What is the infinitive of the verb شست and what are its stems?

شست is the simple past tense for "he/she/it washed". The infinitive is شستن (to wash). Farsi verbs have two stems: the past stem is شست (which you see in this sentence), and the present stem is شوی, which you would use for present actions like meaning "I wash".