Ég er þreyttari í dag en í gær.

Þreyttari is the comparative form of þreyttur, which means tired. So þreyttari means more tired.

A very common pattern in Icelandic is: positive adjective → comparative adjective

For example: þreyttur → þreyttari gamall → eldri (older, irregular) stór → stærri (bigger, irregular)

So in this sentence, þreyttari is the key word that shows comparison.

Because Icelandic usually builds more directly into the adjective itself.

In English, you say more tired. In Icelandic, you say þreyttari.

So -ari is doing the job that more does in English here. That is why you do not need a separate word like meira in this sentence.

Because the main statement is about how the speaker feels today.

The structure is:

I am more tired today than yesterday.

So the speaker’s current state is present, which is why Icelandic uses er = am / is / are.

The phrase í gær is only the comparison point. It does not make the whole sentence past tense.

If the whole situation were in the past, you would use var instead: Ég var þreyttari í gær en í fyrradag. I was more tired yesterday than the day before yesterday.

Here, en means than.

After a comparative adjective, en introduces what you are comparing with: þreyttari ... en ... more tired ... than ...

Be aware that en can also mean but in other sentences, so you have to understand it from context.

Because Icelandic, like English, often leaves out repeated words when the meaning is obvious.

English does this too: I’m more tired today than yesterday. This really means: I’m more tired today than I was yesterday.

Icelandic works the same way here. The missing part is understood from context, so it does not need to be said.

They are very common time expressions:

í dag = today
í gær = yesterday

You should learn them as fixed expressions. The word í often corresponds to in in other contexts, but in these phrases it is just part of the normal Icelandic way to express time.

So it is best not to translate them word by word too literally. Just remember: í dag = today
í gær = yesterday

In this sentence, þreyttari is the form both a male speaker and a female speaker would normally use.

That is different from the basic adjective in the positive degree: A man would say Ég er þreyttur. A woman would say Ég er þreytt.

But in this comparative sentence, both would say: Ég er þreyttari í dag en í gær.

So this is a useful thing to notice: the comparative does not behave exactly like the basic form.

Yes. You can also say:

Í dag er ég þreyttari en í gær.

That is perfectly natural. It puts extra focus on today.

But if you move í dag to the front, the verb still has to come second: Í dag er ég ...

Not: Í dag ég er ...

This is an important Icelandic word-order rule called verb-second word order.

Normally, no. With an adjective like þreyttur, Icelandic uses the comparative adjective: þreyttari

So the natural sentence is: Ég er þreyttari í dag en í gær.

Using meira here would sound wrong or unnatural to most learners’ target grammar. For ordinary comparison of adjectives, use the comparative form itself.

The letter þ is pronounced like the th in thing, not like the th in this.

So þreyttari begins with that voiceless th sound.

A rough English-friendly approximation of þreyttari would be: THRAYT-ta-ri

That is only approximate, but it is a helpful starting point. The important thing is that þ is never pronounced like an English t.