Streitan er mikil í vinnunni í dag.

The dictionary form is streita, meaning stress or strain.

The form streitan means the stress. Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• streita = stress
• streitan = the stress

Because streita is a feminine noun ending in -a, the definite nominative singular form becomes -an.

Because Icelandic commonly uses a suffixed definite article.

Instead of saying something like the stress with a separate word, Icelandic usually attaches the to the noun itself:

• streita → streitan
• vinna → vinnan
• vinnu
  • definite dative ending → vinnunni

So in this sentence, both streitan and vinnunni already include the idea of the.

Because the adjective has to agree with streitan.

Here, streitan is:

• feminine
• singular
• nominative

So the adjective mikill appears in its feminine singular nominative form:

• masculine: mikill
• feminine: mikil
• neuter: mikið

Since streitan is feminine, mikil is the correct form.

Literally, mikil can mean big, great, or much/a lot of, depending on context.

With something like stress, the most natural English sense is:

• high
• great
• a lot of

So Streitan er mikil is not really The stress is big in natural English. It is more like:

• The stress is high
• There is a lot of stress

Vinnunni is dative singular definite of vinna.

That happens because the preposition í takes:

• dative when it means in/at a place, with no movement
• accusative when it means movement into something

Here, the meaning is location: at work / in the workplace, not movement. So Icelandic uses the dative:

• í vinnunni = at work / in the workplace

Compare:

• Ég er í vinnunni = I am at work
• Ég fer í vinnuna = I am going to work

Yes. Even though í often looks like in, í vinnunni is a normal Icelandic way to say at work.

Literally, it is closer to in the work or in the workplace, but idiomatically the meaning is simply:

• at work

This is a good example of how prepositions do not match perfectly from one language to another. English says at work, but Icelandic uses í here.

Because í dag is a fixed idiomatic expression meaning today.

Even though learners often expect í to behave the same way everywhere, time expressions can be more idiomatic. So í dag should be learned as a set phrase.

Just memorize:

• í dag = today

It does not work like a simple, regular location phrase that you can always analyze the same way as í vinnunni.

Because this sentence uses a copula construction: noun + er + adjective.

So:

• Streitan = the subject
• er = is
• mikil = predicate adjective describing the subject

This works just like English in sentences such as:

• The stress is high
• The weather is good

So mikil is not directly sitting before the noun; it is linked to the noun through er.

Yes. Icelandic word order is flexible, but it follows the V2 rule in main clauses, meaning the finite verb usually stays in the second position.

So you can move parts of the sentence for emphasis:

• Streitan er mikil í vinnunni í dag.
• Í dag er streitan mikil í vinnunni.
• Í vinnunni er streitan mikil í dag.

These all mean roughly the same thing, but the first element gets more emphasis.

No. Another very natural sentence would be:

• Það er mikil streita í vinnunni í dag.

That means:

• There is a lot of stress at work today.

The version you were given, Streitan er mikil í vinnunni í dag, puts the stress itself in focus as the subject. The Það er... version presents it more as a general situation.

Both are good Icelandic, but they feel slightly different in emphasis.