In the phrase above you can find what is called a comparative: langzamer. The original form langzaam is extended wit -er and because of the Dutch spelling rules (lesson 1) there's one -a- less.
Except for the one -a- thing, this is the same as what happens in English: 'thin-thinner' is in Dutch dun-dunner and even irregular forms are a lot alike: 'good-better' is in Dutch goed-beter.
And there's more good news, for the superlative is almost the same: in English you add '-est' ('thinnest'), in Dutch just -st (dunst). Usually the article het is used for the superlative (Dit boek is het dunst).
The only tricky thing, is that you have an extra -d if the word ends on a -r
More good news: the list of irregular ones is very short:
Here's a list of examples, which is at the same time a list of opposites: