Umferðin er meiri í dag en í gær.

Umferð means traffic. The ending -in is the definite article attached to the noun, so umferðin means the traffic.

This is very common in Icelandic: instead of a separate word for the, the article is often added to the end of the noun.

• umferð = traffic
• umferðin = the traffic

It is singular. Umferð is a singular mass noun, like English traffic.

So even though traffic involves many cars, the grammar treats it as one singular idea:

• Umferðin er ... = The traffic is ...

That is why the verb is er = is, not a plural verb.

Because meiri has to match umferðin grammatically.

The noun umferð is feminine singular, and the comparative adjective agrees with it. So here Icelandic uses meiri.

The basic comparison is:

• mikill = much / great / large
• meiri = more / greater

Meira is the neuter singular form, and it is also often used adverbially, so it does not fit this noun here.

The dictionary form is mikill.

This adjective often means much, big, great, or large, depending on context. Its comparison forms are irregular enough that learners usually memorize them as a set:

• mikill = much / great
• meiri = more / greater
• mestur = most / greatest

In a sentence about traffic, meiri is usually best understood as more or heavier.

Here, en means than.

Icelandic en can mean both but and than, depending on context. After a comparative word like meiri, it means than:

• meiri ... en ... = more ... than ...

So:

• meiri í dag en í gær = more today than yesterday

Because these are fixed time expressions:

• í dag = today
• í gær = yesterday

It is best to learn them as whole expressions. Although í often means in, in these phrases the meaning is idiomatic rather than fully literal.

A few related time expressions work similarly:

• í morgun = this morning
• í kvöld = tonight / this evening

The sentence breaks down like this:

• Umferðin = the traffic
• er = is
• meiri = more / greater
• í dag = today
• en í gær = than yesterday

So the literal structure is close to:

The traffic is greater today than yesterday.

In natural English, you might also say:

There is more traffic today than yesterday.

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

So en í gær really means something like:

• than it was yesterday
• than yesterday
• than yesterday’s traffic level

The full idea is understood from the first part of the sentence, so it does not need to be repeated.

Yes. That is perfectly natural.

Both of these work:

• Umferðin er meiri í dag en í gær.
• Í dag er umferðin meiri en í gær.

The second version puts more focus on today.

It also shows a very important Icelandic pattern: when you move something to the front, the finite verb still comes in second position. So you get:

• Í dag er ...

not

• Í dag umferðin er ...

Yes, absolutely. That is also a very common and natural sentence.

There is a slight difference in feel:

• Umferðin er meiri í dag en í gær = The traffic is heavier today than yesterday
• Það er meiri umferð í dag en í gær = There is more traffic today than yesterday

The version with það er often sounds a bit more like introducing a fact.
The version with umferðin can sound a bit more like you are talking about the traffic situation itself.

Both are good Icelandic.