All Islandish verbs can be inflected in three different aspects:
perfective
imperfective
habitual
If the speaker uses a perfective aspect, he sees the action as a single point in time, which cannot be further divided and doesn't have any duration.
Eg.
Stak nej jabrzedl. The train has arrived.
The present perfective refers to a moment in the future:
Šelknem Querítelu. I will start living in Querítel. / I will move to Querítel.
The use of imperfective aspect presents the action as stretching across some time period, which, usually, has a beginning or an end, but doesn't need to. During the course of the expressed action other things can happen, or it can be interrupted by something else.
Eg.
Stak nej jabrzel. The train was arriving.
Grál nej čerň hoka nej glídl mněrkeré spáriňí. He was reading a book when he heard cries for help. (Note the interrupted action is in imperfective, the interupting one in perfective.)
The habitual aspect expresses an action, which takes place repeatedly or a series of actions which take place one after the other.
Eg.
Staci nej jabrzevli. The trains have been coming.
Nerí baba nej lúvl i plešovu. My grandpa used to swim in this lake.
The citation form of a verb is always the imperfective infinitive. The perfective is marked with a -s- infix before the -ti infinitive suffix and a -d- in the conjugation. The habitual is marked with a -va- infix before the -ti infinitive suffix and a -j- or -v- in the conjugation. Please see the conjugation tables in the article about Islandish verbs.