Jag måste fylla i blanketten idag.

Because fylla i is a fixed verb phrase, a particle verb. In this sentence it means to fill in / fill out.

• fylla by itself usually means to fill
• fylla i means to fill in, especially a form, questionnaire, or blank space

So:

• fylla ett glas = fill a glass
• fylla i en blankett = fill in a form

The little word i is important here.

Blanketten is the definite singular form of blankett, meaning the form.

Swedish often adds the definite article to the end of the noun instead of using a separate word like English the.

Here are the forms:

• en blankett = a form
• blanketten = the form
• blanketter = forms
• blanketterna = the forms

So blanketten literally contains the idea of the.

Because Swedish usually puts the definite article on the noun itself as a suffix.

In English:

• the form

In Swedish:

• blanketten

This is very normal in Swedish. You will see it all the time:

• boken = the book
• huset = the house
• dagen = the day

So blanketten already means the form, and no extra word is needed.

Because måste is a modal verb, and modal verbs are followed by the bare infinitive in Swedish.

So:

• Jag måste fylla i blanketten.
• literally: I must fill in the form.

This is similar to English:

• I must fill in the form not
• I must to fill in the form

After måste, you use the infinitive form of the main verb:

• måste gå = must go
• måste läsa = must read
• måste fylla i = must fill in

For the same reason: after a modal verb like måste, Swedish does not use att.

Compare:

• Jag vill läsa. = I want to read.
• Jag måste läsa. = I must read.

Not:

• Jag måste att läsa

So after måste, just use the infinitive directly.

In the present tense, no. Måste stays the same for all subjects.

• jag måste = I must
• du måste = you must
• han måste = he must
• vi måste = we must

This is easier than English verbs like am/is/are because Swedish verbs do not change for person in the present tense.

Yes, idag can move.

In this sentence, putting idag at the end is a very natural, neutral word order:

• Jag måste fylla i blanketten idag.

But you can also put it first for emphasis:

• Idag måste jag fylla i blanketten.

That means something like Today, I have to fill in the form.

A useful rule for Swedish main clauses is that the finite verb usually comes in the second position. So when idag moves to the front, måste still has to stay second:

• Idag måste jag ...

not

• Idag jag måste ...

Usually, yes: idag.

You may also sometimes see i dag written as two words. Both are accepted, but idag is very common in modern Swedish.

So for a learner, idag is a perfectly good form to use.

Blankett is an en word, also called a common gender noun.

That is why the indefinite form is:

• en blankett = a form

And the definite form becomes:

• blanketten = the form

If it were an ett word, the definite singular would usually end differently, often in -et instead of -en.

The straightforward negative form is:

• Jag måste inte fylla i blanketten idag.

But be careful: this can sometimes sound like I must not fill in the form today or It isn’t necessary for me to fill in the form today, depending on context.

If you want to say I don’t have to fill in the form today, Swedish often prefers:

• Jag behöver inte fylla i blanketten idag.

So:

• måste inte can be ambiguous
• behöver inte is often clearer for don’t have to

A careful pronunciation would be roughly:

Yah must-eh FIL-la ee BLAN-ket-ten ee-DAH

A few helpful notes:

• Jag is often pronounced more like yah than the spelling suggests.
• måste has a short å sound.
• fylla has a front rounded y sound, which English does not really have.
• Stress usually falls like this:
  • JAG måste FYLLa i blanketten idag
  • with the strongest stress often on fylla or idag, depending on emphasis

If you want very natural rhythm, try saying:

Jag måste fylla i blanketten idag.

smoothly, without stressing every word equally.