Learn Russian: Мальчик одевается очень медленно.

The -ся ending makes the verb reflexive, which usually means the action is done to oneself. Without it, одевать means "to dress (someone else)". Adding -ся turns it into "to dress oneself" or "to get dressed".

Why does this verb use -ся instead of -сь?

You choose between -ся and -сь based on the last letter of the conjugated verb. The "he" form is одевает. Because it ends in a consonant (т), we add -ся. If the form ended in a vowel (like the "I" form, одеваюсь), we would add -сь instead.

How do you pronounce the -тся at the end of одевается?

Even though it is spelled -тся, it is pronounced like -ца ("tsa"). The "т" and "с" merge together to make a single "ts" sound, and there is no soft sound at the end.

Why does медленно end in -о?

Words ending in -о are often adverbs, which describe how an action is done. Just like adding "-ly" to "slow" makes "slowly" in English, taking the adjective медленный (slow) and changing the ending to -о gives us the adverb медленно (slowly).