Learn Hardcore Icelandic: Þessi æfing er erfiðari en æfingin í gær. - This exercise is more difficult than the exercise yesterday.

The first er is the verb “is” (present tense of vera = to be).
The second er is just the first syllable of the adjective erfiðari (more difficult).

So grammatically it’s:

er (is)
erfiðari (more difficult)

It only looks like “er er” because erfiðari starts with the letters er-. You must keep the verb er; you cannot say Þessi æfing erfiðari en… — that would be ungrammatical.

What is the base form of erfiðari, and how is this comparative formed?

The base (positive) form is erfiður = difficult.

Degrees of comparison:

erfiður – difficult
erfiðari – more difficult (comparative)
erfiðastur – most difficult (superlative)

Formation:

Take the stem erfið-
Add -ari → erfiðari

You use the comparative erfiðari with en to compare two things:

Þessi æfing er erfiðari en æfingin í gær.
This exercise is more difficult than the exercise yesterday.

What case, gender, and number is æfing in here, and why?

In Þessi æfing er erfiðari…:

æfing is nominative, singular, feminine.

Reasons:

It’s the subject of the sentence → subjects are in the nominative.
The dictionary form æfing is feminine; it’s used here without the definite ending, so it appears exactly as in the dictionary.

The second æfingin (with -in) is also nominative singular feminine, but definite (the exercise). It is the thing being compared to.

Why is it æfing first and æfingin later? Why does the second one have -in?

Icelandic usually shows “the” by adding a definite ending to the noun:

æfing = exercise (an exercise)
æfingin = the exercise

In this sentence:

Þessi æfing already has Þessi (this), so you don’t need the definite ending on æfing; Þessi makes it specific.
æfingin í gær means the exercise yesterday — a specific, known exercise (yesterday’s one), so the definite ending -in is used.

You could also say:

Æfingin í dag er erfiðari en æfingin í gær.
The exercise today is more difficult than the exercise yesterday.

There both are definite.

Why is it en here? Is that the same word as en meaning “but”?

Yes, the word en has two main uses:

en = but (conjunction)
en = than after comparatives

In this sentence it is the “than” use:

erfiðari en… = more difficult than…

So:

Þessi æfing er erfiðari en æfingin í gær.
This exercise is more difficult than the exercise yesterday.

Why isn’t there a word like “more” in Icelandic? In English we say “more difficult than…”.

In Icelandic, the idea of “more” is usually built directly into the comparative form of the adjective:

erfiður = difficult
erfiðari = more difficult
gamall = old
eldri = older / more old

So Icelandic doesn’t normally say “meira erfiður” (more difficult); it just changes the adjective to a comparative form like erfiðari. The combination erfiðari en X already means more difficult than X.

Does erfiðari change for gender, number, or case here?

Comparative adjectives do decline, but less visibly than the positive forms.

Relevant pattern:

Masculine/feminine nominative singular: erfiðari
Neuter nominative singular: erfiðara

So:

Hann er erfiðari. – He is more difficult.
Hún er erfiðari. – She is more difficult.
Verkið er erfiðara. – The task is more difficult. (neuter)

In Þessi æfing er erfiðari…, æfing is feminine, nominative singular, so erfiðari is the correct form.

Why is the time expression í gær used with í? Why not just gær?

You can actually say í gær or just gær in some contexts, but í gær is the normal, idiomatic way to say yesterday in a sentence like this.

í is a preposition that often means in / on / at.
With gær, it forms a frozen time expression í gær = yesterday.

Case-wise:

í
- a time word like gær → gær works as an adverbial of time; you don’t see case endings here, but you normally learn the phrase as a fixed unit: í gær = yesterday.

Why is the verb er in the present tense when we’re talking about something compared to yesterday?

The tense refers to when the sentence is true, not to the comparison point.

Þessi æfing er erfiðari en æfingin í gær.
means: Right now, this exercise is more difficult than the exercise was yesterday.

So:

er = is (now)
The phrase í gær just tells you when the other exercise took place, not the tense of the verb. The past is already expressed by í gær (yesterday).

Can I drop the second æfingin and just say Þessi æfing er erfiðari en í gær?

Yes, that is possible, but the meaning changes slightly.

Þessi æfing er erfiðari en æfingin í gær.
→ compares this exercise with the exercise yesterday.
Þessi æfing er erfiðari en í gær.
→ more like: This exercise is more difficult than (it was) yesterday or
Training/practice is more difficult than yesterday (more vague; “what” is understood from context).

The original sentence explicitly compares one exercise to another exercise. Dropping æfingin makes it more general or contextual.

Could I use a pronoun instead of repeating æfingin? For example, something like “than that one yesterday”?

Yes. A natural way is:

Þessi æfing er erfiðari en sú í gær.

Here:

sú is the feminine nominative singular form of the demonstrative sá / sú / það meaning roughly that one.

So the sentence becomes: This exercise is more difficult than the one yesterday.

What is the pronunciation of Þessi æfing er erfiðari en æfingin í gær?

Very roughly (English-friendly approximation):

Þessi ≈ THESS‑i
- Þ like th in thin.
æfing ≈ EYE‑vingk
- æ like eye.
er ≈ air (shorter)
erfiðari ≈ AIR‑vith‑a‑ri
- ð like voiced th in this, but often quite soft.
en ≈ en (like hen without the h)
æfingin ≈ EYE‑ving‑in (with a light n)
í ≈ long ee (like see)
gær ≈ gyehr
- g is hard at the start,
- æ = eye,
- r is rolled or tapped.

Spoken fluently, some sounds blend together, but this gives you a useful approximation.