Lithuanian has a lot of prefixes that you can attach to various parts of speech. So far you have seen only two prefixes that you can add in front of a verb: ne and te. Both of them can be considered verbal prefixes, but there are twelve more verbal prefixes that you will encounter in the course of your Lithuanian studies:

ap-
at-
į-
iš-
nu-
pa-
par-
per-
pra-
pri-
su-
už-

When added to a verb, these prefixes considerably change the meaning of the verb's action.

Without a prefix, verbs express a continuous action (in some languages, this is called the imperfect aspect). But when a prefix is added, the action of the verb becomes momental, finished or done once (aka the perfect aspect).

Let's take the word eiti for example. Without a prefix, it means the continuous action of going (walking), or the process of going (walking).

When we add a prefix like pa to it (paeiti), it then means going (walking) for a short time or finished going (walking).

Pa is the most common prefix of all the twelve. Many verbs do not even have prefixes other than pa.

Other prefixes have a more specific meaning. For example, į means a direction inwards. When we add it to eiti, įeiti means going (walking) inwards:

Aš įeinu į kambarį. I go into the room (this action is made once and for all).
Aš einu į kambarį. I am going into the room (the process of this action).

Prefix iš means a direction outwards. When we add it to eiti, išeiti means going (walking) outwards:

Aš išeinu iš kambario. I go out of the room (this action is made once and for all).
Aš einu iš kambario. I am going out of the room (the process of this action).