Learn Spanish (Latin America): El niño mira la puerta negra.

In Spanish, the verb mirar means "to look at" or "to watch." The English word "at" is already built into the meaning, so you do not need to add an extra word for it.

Why is the word for black (negra) placed after the door (puerta)?

In Spanish, descriptive words (adjectives) usually come after the thing they describe. So instead of the English word order, the sentence essentially says the door black (la puerta negra).

Why is it negra instead of negro?

Adjectives must match the gender of the noun they describe. Because puerta is a feminine noun—which we can see from the word la—the adjective negro changes to its feminine form, negra.

Since the current lesson is about estar, could we use it here to say the boy is looking?

Yes, you can use estar to say está mirando (he is looking right now). However, Spanish uses the simple present tense (mira) much more often than English to mean both he looks and he is looking.

If we wanted to say the door is black, would we use ser or estar?