Learn Hardcore Icelandic: Ég drekk yfirleitt bara einn bolla af kaffi.

It’s accusative singular. The verb drekka takes an accusative object, so masculine weak noun bolli becomes bolla in the accusative. With the numeral, the noun still follows the case required by the verb: (ég) drekk einn bolla.

Why is it af kaffi? Which case does af take?

Af governs the dative. Kaffi (neuter) has the same form in nominative and dative singular, so you see no change. The pattern for measures is container + af + dative:

bolli af kaffi (cup of coffee)
glas af víni (glass of wine)
skeið af sykri (spoon of sugar)

Could I say einn kaffibolla instead of einn bolla af kaffi?

Yes, very common. Kaffibolli is a compound and usually means “a cup of coffee” (the drink).

einn bolla af kaffi = emphasizes the measure “a cup of coffee (as a quantity)”
einn kaffibolla = the standard everyday way to refer to one cup of coffee

Is einn here the numeral “one,” or is it acting like the English article “a”?

It’s the numeral “one.” Icelandic has no indefinite article. You use einn/ein/eitt when you really mean the number one, and here it pairs naturally with bara “only” to mean “only one.”

Why is it einn bolla and not eina bolla or eitt bolla?

Agreement. Bolli is masculine, accusative singular, so the numeral must match: masculine accusative einn.

Masculine: einn (acc.)
Feminine: eina (acc.)
Neuter: eitt (acc.)

Can I move the adverbs around? Why are yfirleitt and bara in those positions?

Icelandic is verb-second: the finite verb (drekk) is in the second slot.

Yfirleitt is a sentence adverb (“usually”) and fits well right after the verb or in first position: Yfirleitt drekk ég…
Bara should sit right before what it limits; here it limits the quantity, so it goes before einn bolla. Avoid Ég drekk bara yfirleitt… unless you mean “I only usually drink…”, which is odd.

Could I use venjulega instead of yfirleitt?

Yes. Both often mean “usually.” Nuance:

yfirleitt = “generally/by and large”
venjulega = “as a rule/in the usual way” They overlap here: Ég drekk venjulega bara einn bolla…

What’s the difference between bara, aðeins, and einungis?

All can mean “only,” but register differs:

bara: very common, informal, also “just/simply”
aðeins: neutral to slightly formal; can also mean “slightly”
einungis: formal/literary All three work here.

How do I say “two/three/four cups of coffee” in this structure?

Use the masculine accusative forms of the numerals and the accusative plural bolla:

2: tvo bolla af kaffi
3: þrjá bolla af kaffi
4: fjóra bolla af kaffi

Why not use a progressive form like “I am drinking”? Should it be Ég er að drekka?

For habits, Icelandic uses the simple present: Ég drekk…. The er að + infinitive form is for ongoing actions right now: Ég er að drekka = “I’m drinking (now).”

Why doesn’t kaffi change after af? Shouldn’t I see an ending?

Because kaffi is one of those neuter nouns whose dative singular looks the same as nominative: kaffi. You would see a change with nouns like vín → víni, sykur → sykri.

Can I just say “one coffee” like in a café, i.e., eitt kaffi?

Yes, colloquially when ordering or counting servings: eitt kaffi, tvö kaffi. In more careful phrasing about quantity, use the container: einn (kaffi)bolla.

Is af the only choice here? What about úr?

Use af for “a measure of” constructions. Úr means “out of/from inside” and isn’t used for quantities:

Correct: bolli af kaffi
Not for quantity: ✗ bolli úr kaffi

How would this sentence look in the past tense?

Change the verb to past: Ég drakk yfirleitt bara einn bolla af kaffi.

Quick declension check: how does bolli decline in the forms we need?

Key forms:

Singular: nominative bolli, accusative bolla, dative bolla
Plural: nominative bollar, accusative bolla, dative bollum So: einn bolla, tvo/þrjá/fjóra bolla, úr bollum (from cups).