Adverbs that end in -ai and -iai can be compared as in 'much, more, the most' or 'strongly, more strongly, the most strongly'.

Comparative degree

In order to make an adverb into the comparative degree (i.e. more strongly), you have to drop the ending -ai or -iai and add -iau.

Greitai (quickly) - greičiau.
Labai (very) - labiau.
Blogai (badly) - blogiau.
Gražiai (nicely) - gražiau.
Šlapiai (wetly) - šlapiau.
Žaliai (greenly) - žaliau.

Notice that the adverbs, which have a letter t in the end (greitai), change it into č (greičiau).

Jonas bėga greitai, bet Petras greičiau. Jonas runs quickly, but Petras - more quickly.
Ji labai stengiasi, tačiau galėtų stengtis labiau. She tries very hard, however she could try harder.
Ji atrodo blogiau nei vakar. She looks worse than yesterday.
Jis dainuoja gražiau negu vakar. He sings nicer than yesterday.

Just like in comparing adjectives, the little words are už, nei and negu, meaning 'than'.

Superlative

An adverb is made into the superlative (i.e. the most strongly) by adding a -iausiai ending instead of just -iau.

Greitai (quickly) - greičiau - greičiausiai.
Labai (very) - labiau - labiausiai.
Blogai (badly) - blogiau - blogiausiai.
Gražiai (nicely) - gražiau - gražiausiai.
Šlapiai (wetly) - šlapiau - šlapiausiai.
Žaliai (greenly) - žaliau - žaliausiai.

Jis bėga greičiausiai. He runs the most quickly.
Aš labiausiai stengiuosi. I try the hardest.

You may revise the comparison of adjectives again or just pick some more grammar from the lesson list.