Verbal Aspect

The Czech verbs are distinguished by aspect. They are either perfective or imperfective.

Imperfective verbs (czech: nedokonavý vid)
The imperfective verbs describe an unfinished or repeated action, the duration is not specified.

pít - to drink, to be drinking
jíst - to eat, to be eating

Equivalent in English could be continuous tenses.
Já piju. (I am drinking.)
Já jím. (I am eating.)
Budu pít. (I will be drinking.)
Já jím. (I am eating.)
Pil jsem džus. (I was drinking juice.)
Jedl jsem sušenku. (I was eating a biscuit.)

Perfective verbs (czech: dokonavý vid)
The perfective verbs indicate the finality of the process.
They describe on-time, finished actions within a specific time frame.
They cannot express the present tense.

napít - to have a drink
vypít - to drink up, to finish drinking
najíst - to have a meal
sníst - to eat up, to finish eating

Equivalent in English could be the simple past or simple future tense.

On se napil. (He drank, meaning he's not thirsty anymore)
Ona vypila džus. (She drank that juice, meaning she drank it whole, nothing is left.)
On se najedl. (He ate, meaning he's not hungry anymore)
Ona snědla sušenku. (She ate a biscuit, meaning she ate it whole, nothing is left.)

Napiju se toho džusu. (I will drink that juice, meaning I will make myself not thirsty by drinking some of that juice.)
On sní tu sušenku. (He will eat that biscuit, meaning he will eat it whole.)

Forming perfective and imperfective verbs:
The Perfective verbs are often formed by adding prefixes, such as s-, za-, při-, u-, vy-, na-, do-, to imperfective verbs.
The Imperfective verbs are often formed by changing the verb stem into -ova-, -va-, -ěva-.
However, there is no a straightforward rule specifying how to form perfective and imperfective verbs.

Perfective - Imperfective pairs (=Vidová dvojice)
There are usually two verbs that differ only in verbal aspect. They create so-called "vidová dvojice".
However, sometimes two completely different verbs are used to express perfectiveness and imperfectiveness.

Examples of regular Perfective - Imperfective pairs:
dojít - jít (to come - to go, to be going)
dojíst - jíst (to finish eating - to eat, to be eating)
doříct - říct (to be talking, to finish talking - to talk)

Examples of irregular perfective - imperfective pairs:
říct - říkat (to tell, to tell everything - to tell, to be telling)
Řekli jsme mu o té knize. (We told him about the book.)
Pořád mi říká, co mám dělat. (He keeps telling me what to do.)

koupit - nakupovat (to buy - to be shopping)
Marie koupila knihu. (Maria bought a book.)
Marie byla nakupovat. (Maria went shopping.)

přečíst - číst (to finish reading - to be reading)
Přečetl jsem si tu knihu. (I read that book.)
Čtu tu knihu. (I am reading that book.)

najít - hledat (to find - to search to look for, to be searching, to be looking for)
Našla jsem moji peněženku. (I have found my wallet.)
Hledám moji peněženku. (I am looking for my wallet.)

There is often more than one perfective and imperfective form of the same verb.
Each form usually implies a subtle difference in meaning.

More examples of Perfective - Imperfective pairs:
psát (to write)
napsat - psát (to write something)
odepsat - odepisovat (to reply in writing or to write off)
připsat - připisovat (to add by writing)
přepsat - přepisovat (to change by writing, to write over)
vepsat - vepisovat (to insert by writing)
etc...

přijít (to come)
přijít - přicházet (to come, to arrive)
odejít - odcházet (to leave)
nadejít si - nadcházet si (to take the long way)
podejít - podcházet (to walk under)
přejít - přecházet (to cross, to walk across)
ujít - ucházet (to cover a distance by walking)
vyjít - vycházet (to come out(side))

koupit (to buy)
koupit - kupovat (to buy something)
odkoupit - odkupovat (to buy from)
přikoupit - přikupovat (to buy more of the same thing)
vykoupit - vykoupovat (to buy everything out)
zakoupit - zakupovat (to purchase)
etc...