Learn Spanish (Latin America): El niño abre la boca.

In Spanish, you use definite articles (el, la, los, las) for body parts instead of possessive words like mi or su. Since we know the boy is doing the action, it is obvious to a Spanish speaker that he is opening his own mouth.

What is the infinitive (dictionary form) of abre?

The verb is abrir (to open). It is a regular -ir verb, so for el niño (he), you drop the -ir and add -e to get abre.

How does knowing to use la boca help with verbs like doler (to hurt) from this lesson?

Verbs like doler follow this exact same rule for body parts! If the boy's mouth hurt, you would say Le duele la boca (Literally: The mouth hurts to him). Just like in this sentence, you use the article la instead of the possessive su.