Learn Spanish (Latin America): ¿Por qué está riendo el bebé?

Reír has an irregular gerund. For verbs ending in -eír (like reír to laugh, and sonreír to smile), the e changes to an i. To avoid having an awkward double 'i' sound, it simply becomes riendo instead of "riiendo" or "reiendo".

Could I just say "¿Por qué ríe el bebé?" instead of using está riendo?

Yes, you could! In Spanish, the simple present tense (ríe) can be used for actions happening right now. However, using the present continuous (está riendo) strongly emphasizes that the baby is in the middle of laughing at this exact moment.

Why is el bebé at the very end of the sentence?

In Spanish questions, it's very common to put the subject (el bebé) after the verb. While English splits the verb phrase in questions ("Why is the baby laughing?"), Spanish usually keeps the two-part verb (está riendo) together and places the subject right after it.

Why is Por qué written as two words with an accent mark here?

When written as two separate words with an accent on the é, por qué means "why" and is used for questions. If you write it as one word without an accent (porque), it means "because".