Learn Hardcore Icelandic: Drykkurinn kostar minna en ég hélt.

In Icelandic the definite article isn’t a separate word but a suffix attached to the noun. Drykkur means “a drink,” and -inn makes it definite: drykkurinn = “the drink.”

Why is the comparative minna used here? Shouldn’t an adjective agree with drykkurinn in gender and case?

Here minna is an adverb modifying the verb kostar (“costs”). Adverb comparatives in Icelandic are indeclinable (always ending in -a). You use minna to say “costs less.” If you wanted an adjective agreeing with drykkurinn, you’d say ódýrari (“cheaper”) and make it:
Drykkurinn er ódýrari en ég hélt.

What role does en play in minna en ég hélt?

In comparisons en means “than.” So minna en ég hélt = “less than I thought.” (Note: en can also mean “but” in other contexts, but here it’s the comparative “than.”)

Why is there no að before ég hélt?

The full sentence could be Drykkurinn kostar minna en ég hélt að hann myndi kosta, but in everyday speech Icelanders drop the subordinate clause and the að. So en ég hélt is a shortened way of saying “than I had thought.”

Why is hélt in the past tense? Could I use present held instead?

Using past tense hélt emphasizes that this was your expectation before seeing the price. If you say en ég held, it means “than I think (right now).” Both are correct; they just shift the time reference of your thought.

Why is the verb kostar in this form? Which person and number is it?

Kostar is the present tense, third person singular of að kosta (“to cost”). It matches the singular subject drykkurinn.

Can I drop Drykkurinn and just say Kostar minna en ég hélt?

Yes. In conversational Icelandic, if the context is clear, you can omit the subject: Kostar minna en ég hélt still means “It costs less than I thought.”

How would you say “The drinks cost less than I thought”?

Use the definite plural drykkirnir and plural verb kosta:
Drykkirnir kosta minna en ég hélt.