An adverb explains a verb or an adjective:

Jis bėga (verb) greitai (adverb). He runs quickly. Jis labai (adverb) gražus (adjective). He is very handsome.

Most adverbs are made of adjectives. Such adverbs can either have the ending -ai or -iai. Unlike adjectives or nouns, adverbs don't have cases or whatsoever.

-ai adverbs

These adverbs are formed from adjectives of the first declension which have the endings -as, -a. For example, geras, gera (good) can be made into an adverb gerai (well). Džiaugsmingas, džiaugsminga (glad) can be made into džiaugsmingai (gladly).

-iai adverbs

These are made from the first declension's adjectives with the ending -ias, -ia. For example, šlapias, šlapia (wet) can be made into šlapiai ('wetly'). Moreover, adjectives of the second declension ending in -us, -i are also formed into an -iai adverb. For example, platus, plati (wide) is formed into plačiai (widely).

Remember that you can't form adverbs from adjectives of the third declension (which often end in -inis).

You can compare adverbs just like adjectives. See adverb comparison or revise adjective comparison.

Besides -ai and -iai adverbs, there are a lot of other adverbs, which do not end in -ai or -iai. Good examples are the following: kitaip (otherwise), arti (near), etc.

Sorts of Adverbs

According to meaning, adverbs can be divided into these five sorts:

Temper adverbs: gražiai (nicely), staiga (suddenly), kitaip (otherwise).
Quantity adverbs: maždaug (approximately), trissyk (thrice), dviese (double; to do something by two people).
Place adverbs: kur ne kur (in random places; somewhere), čia (here), nuošaliai (aside), arti (near).
Time adverbs: vakar (yesterday), tąsyk (that time), niekada (never), seniai (long ago).
Reason adverbs: tyčia (deliberately), kažkodėl (for some reason), dėl to (because of that).

Let's see how all the previous adverbs act in sentences.

Kaip gražiai jie atrodo! How pretty they look!
Staiga pamačiau jo šešėlį. Suddenly I saw his shadow.
Tu atrodai visiškai kitaip nei tave įsivaizdavau. You look absolutely different than the way I have imagined you.
Kelionė į Latviją kainuoja maždaug penkiasdešimt litų. A trip to Latvia costs approximately fifty Lt.
Skambinau trissyk, bet neatsiliepei. I called you three times, but you didn't answer.
Šiandien mes dviese buvome susitikime. Today we (two of us) were in a meeting.
Tiktai kur ne kur gali rasti grybų miške. Only in some places can you find mushrooms in the forest.
Čia mano namai. Here is my home.
Laikykis nuošaliai. Keep away.
Jis gyvena arti mano miesto. He lives near my town.
Mačiau tave vakar. I saw you yesterday.
Tąsyk tau pasisekė. At that time you were lucky.
Never say never. Niekada nesakyk niekada.
Seniai seniai buvo drakonas ir princesė... Once upon a time, there were a dragon and a princess...
Tu tai tyčia pasakei. You said it deliberately.
Kažkodėl aš tavimi nepasitikiu. For some reason I don't trust you.
Aš buvau užsiėmusi, dėl to (todėl) negalėjau ateiti pas tave. I was busy, for this reason (therefore) I could not come to you.

Quantity Adverb

Previously mentioned 'dviese' is a quantity adverb. English doens't have an equivalent for quantity adverbs. They can be described as adverbs, designating that an action is being done by two people (dviese), three people (trise) or more. The highest number of subjects is 9 (devyniese). Of course, the subjects can be anything else, not just people. Let's see them in sentences.

Dviese ėjom pas direktorių. We two (two of us) went to the manager.
Mašinoje buvome trise. We three were in the car.
Keturiese lengviau. It's easier to be four.
Penkiese važiavome į Iraką. We five went to Iraq.
Šešiese, septyniese, aštuoniese ar devyniese - man nesvarbu, tik nueikit į parduotuvę. Six, seven, eight or nine of you - I don't care, just go to the shop.

You may now want to see adverb comparison.