Fakturaperioden börjar idag.

Because -en is the definite singular ending for this noun. It means the.

So:

• en fakturaperiod = a billing period / an invoice period
• fakturaperioden = the billing period / the invoice period

In Swedish, the definite article is often attached to the end of the noun instead of standing as a separate word like English the.

Yes. Swedish very often makes compound nouns by joining words together.

Here, fakturaperioden is built from:

• faktura = invoice
• period = period
• -en = the

So the base compound is fakturaperiod, and the definite form is fakturaperioden.

This is very normal in Swedish. English often writes similar ideas as two words, but Swedish usually prefers one compound word.

Börjar is the present tense of the verb börja, which means to begin or to start.

Some useful forms are:

• börja = to begin
• börjar = begins / is beginning / starts
• började = began
• börjat = begun

In this sentence, börjar means begins or starts.

Because Swedish does not need an extra helping verb here.

English can say:

• The billing period begins today
• The billing period is beginning today
• The billing period does begin today

Swedish often just uses the simple present:

• Fakturaperioden börjar idag.

So börjar already carries the main idea. No extra word like is or does is needed.

Yes, it can come first.

This sentence uses a very normal neutral word order:

• Fakturaperioden börjar idag.

But Swedish also allows:

• Idag börjar fakturaperioden.

Both are correct. When idag comes first, Swedish follows the verb-second rule, so the verb börjar must come immediately after it.

So you get:

• Idag börjar fakturaperioden.

and not:

• Idag fakturaperioden börjar.

Not here.

Since fakturaperioden already means the billing period, you do not add en or den before it.

So:

• en fakturaperiod = a billing period
• fakturaperioden = the billing period

Using den fakturaperioden would usually change the meaning to something more like that billing period or add special emphasis.

Yes. Both idag and i dag are accepted.

• idag is very common in modern Swedish
• i dag is also correct

So these mean the same thing:

• Fakturaperioden börjar idag.
• Fakturaperioden börjar i dag.

Learners will often see both spellings.

In Swedish, nouns are not capitalized just because they are nouns.

So fakturaperioden is lowercase unless it starts the sentence. It is capitalized here only because it is the first word:

• Fakturaperioden börjar idag.

This is different from German, where all nouns are capitalized.

Usually not, if you mean a specific billing period.

Fakturaperiod börjar idag sounds incomplete or unnatural in ordinary Swedish, because bare singular count nouns are not normally used that way.

You would normally say either:

• En fakturaperiod börjar idag = a billing period begins today
• Fakturaperioden börjar idag = the billing period begins today

So in most normal situations, you need either the indefinite form en fakturaperiod or the definite form fakturaperioden.