Ég geymi ber í frystinum, en ég frysti ekki mjólk.

They are geyma and frysta.

In the sentence, geymi and frysti are both 1st person singular present tense forms, so they mean I keep/store and I freeze.

They are related, but not the same:

• geyma = to keep, store
• frysta = to freeze

So in this sentence, the speaker is saying:

• berries are stored/kept in the freezer
• milk is not frozen

That is an important difference. Something can be kept in the freezer without focusing on the action of freezing it.

Here it is understood as plural: berries.

A tricky point is that ber can look the same in both singular and plural in some forms. So the form itself does not always tell you the number. You often need context or the meaning already given.

In this sentence, the intended meaning is clearly berries, not a berry.

They are both direct objects, so they are in the accusative.

• ég geymi ber
• ég frysti ekki mjólk

For these nouns, the accusative form happens to look the same as the basic dictionary form, so there is no visible change here.

Because í takes different cases depending on meaning:

• í + dative = in something, location
• í + accusative = into something, movement

Here the meaning is location: the berries are kept in the freezer, not moved into the freezer. So Icelandic uses the dative.

That is why you get í frystinum.

It shows that the noun is:

• definite: the freezer
• dative singular

The basic noun is frystir = freezer.

So:

• frystir = freezer
• frystinum = the freezer, in a dative context

This is very common in Icelandic: the word for the is usually attached to the end of the noun instead of being a separate word.

Yes.

The difference is usually shown by the case:

• í + dative = in
• í + accusative = into

So:

• í frystinum = in the freezer
• a different accusative form would be used for into the freezer

This is a very important pattern in Icelandic, and also with some other prepositions.

Icelandic does not use do-support like English does.

English says:

• I do not freeze milk

But Icelandic says:

• ég frysti ekki mjólk
• literally, something like I freeze not milk

In a normal main clause, the finite verb comes early in the sentence, and ekki usually comes after that verb. So frysti ekki is the normal order.

En means but.

It joins two contrasting ideas:

• I keep berries in the freezer
• but I do not freeze milk

So it works very much like English but.

Because the sentence contains two full clauses, and each clause has its own verb:

• Ég geymi ber í frystinum
• en ég frysti ekki mjólk

Repeating the subject is completely normal and natural in Icelandic. It makes the structure clear: I do X, but I do not do Y.

Because mjólk here means milk in general, not a specific milk already known in the conversation.

In Icelandic, when you talk about a substance in a general way, you often use the noun without the definite article:

• mjólk = milk
• mjölkin = the milk

So ég frysti ekki mjólk means I do not freeze milk in general.