Pakkinn vegur mikið.

It breaks down like this:

• Pakkinn = the package
• vegur = weighs
• mikið = a lot / much

So the structure is:

subject + verb + adverb

In natural English, that becomes The package weighs a lot.

Because -inn is the suffixed definite article, meaning the.

• pakki = a package
• pakkinn = the package

Icelandic usually puts the at the end of the noun instead of using a separate word before it.

Also, pakkinn here is masculine nominative singular, because it is the subject of the sentence.

The dictionary form is vega, meaning to weigh.

In this sentence, vegur is the present tense, 3rd person singular form, used with a singular subject like pakkinn.

So:

• að vega = to weigh
• pakkinn vegur = the package weighs

Because mikið here is not describing the noun pakkinn. It is modifying the verb vegur.

So in this sentence, mikið functions like an adverb and means a lot / much.

A very common Icelandic pattern is to use the neuter singular form of an adjective adverbially:

• hann vinnur mikið = he works a lot
• pakkinn vegur mikið = the package weighs a lot

So even though pakki is masculine, mikið stays mikið because it is not agreeing with the noun.

Yes. Pakkinn is in the nominative, because it is the subject of the sentence.

This is the thing that is doing the verb action, or more exactly, the thing being described by the verb vega.

Compare:

• Pakkinn vegur mikið = The package weighs a lot
  Here pakkinn is nominative.

If the noun were an object instead, the case could change. For example:

• Ég veg pakkann = I weigh the package

There, pakkann is accusative.

Yes, that is grammatically possible.

• pakki = a package
• pakkinn = the package

So:

• Pakki vegur mikið = A package weighs a lot or possibly a more general statement depending on context
• Pakkinn vegur mikið = The package weighs a lot

The version with pakkinn is definite and refers to a specific package.

Yes, though the basic order here is the most neutral one:

Pakkinn vegur mikið
= The package weighs a lot

This is a normal declarative order: subject + verb + adverb.

Icelandic does allow other word orders for emphasis, but this version is the safest and most natural for a learner to use first.

For a yes/no question, the verb would usually come first:

Vegur pakkinn mikið?
= Does the package weigh a lot?

Yes. Vega can be used both ways:

1. Intransitive: something has a certain weight

   • Pakkinn vegur mikið = The package weighs a lot

2. Transitive: someone measures the weight of something

   • Ég veg pakkann = I weigh the package

So English and Icelandic are similar here: weigh can describe weight, or it can mean measure the weight of.

In Icelandic, the stress is almost always on the first syllable of each word.

So here the stress falls like this:

• PAKK-inn
• VE-gur
• MIK-ið

That first-syllable stress is a very useful general rule in Icelandic pronunciation.