Við kaupum líka spínat og baunir því við viljum elda léttan kvöldmat.

Because the sentence has two clauses:

• Við kaupum líka spínat og baunir
• því við viljum elda léttan kvöldmat

In Icelandic, just as in English, it is normal to state the subject again in the second clause: we buy ... because we want ....

Kaupum is the present tense, 1st person plural form of kaupa, meaning to buy.

So:

• ég kaupi = I buy
• við kaupum = we buy

The ending -um tells you the subject is we.

Líka means also or too. In a simple Icelandic statement, it often comes after the finite verb:

• Við kaupum líka ... = We also buy ...

That placement is very natural in Icelandic. If you move líka, the emphasis can change.

Yes. They are the direct objects of kaupum, and kaupa takes the accusative.

In this sentence:

• spínat is accusative singular
• baunir is accusative plural

For these particular nouns, the accusative forms happen to look the same as the common dictionary forms, so there is no obvious extra ending here.

Here því means because. It introduces the reason:

• því við viljum elda léttan kvöldmat = because we want to cook a light dinner

You may also see því að or af því að for because in other sentences.

After vilja (to want), Icelandic uses the infinitive directly:

• viljum elda = want to cook

So unlike some other Icelandic verb patterns, there is no að here. This is something learners simply need to get used to with vilja.

Because léttan kvöldmat is a direct object, so it must be in the accusative.

The noun kvöldmatur is masculine singular. In the accusative singular, it becomes:

• kvöldmatur → kvöldmat

The adjective must agree with the noun in gender, number, and case, so:

• léttur → léttan

So:

• léttur kvöldmatur = a light dinner/evening meal as a subject form
• léttan kvöldmat = a light dinner/evening meal as an object form

Icelandic does not have a separate indefinite article like English a or an.

So:

• léttan kvöldmat can mean a light dinner

If you want the, Icelandic usually adds the definite article to the end of the noun:

• kvöldmaturinn = the dinner / the evening meal

Because Icelandic very often forms compound nouns.

Kvöldmatur is made from:

• kvöld = evening
• matur = food / meal

So it literally means something like evening meal. In the sentence, it appears in the accusative form kvöldmat.

These are two very important Icelandic letters:

• þ is like th in thing
• ð is like th in this

So:

• því starts with the thing sound
• við contains ð, though at the end of a word its pronunciation can sound softer or less clear than English speakers expect

For learners, the main thing is to remember:

• þ = unvoiced th
• ð = voiced th