Hi. Can you explain me why we have to use more slowly insted of slower. Slow is a One-syllable

Hello, Juan!

Slower is used when comparing nouns directly. Example: Trains are slower than planes.

Whereas, "more slowly" applies when we compare verbs (Different from the verb to be). In the case of the question it is "Trains go _______(slow) than plans". We are making the comparison with the verb "go" and therefore the correct thing is: Trains go more slowly than planes.