Mobilselskabet sender regningen.

Because Danish often puts the at the end of a noun instead of using a separate word in front.

• mobilselskab = mobile company
• mobilselskabet = the mobile company

And:

• regning = bill
• regningen = the bill

So the endings are the definite article suffixes:

• -en for many common-gender nouns
• -et for many neuter nouns

That is why English the appears as -et or -en here.

Because the two nouns belong to different grammatical genders.

• et selskab = a company → neuter noun
  so selskabet = the company
• en regning = a bill → common-gender noun
  so regningen = the bill

Since mobilselskab is a compound noun, it keeps the gender of its final part, selskab.

So:

• et mobilselskab → mobilselskabet
• en regning → regningen

Danish loves compound nouns, just like English does.

Mobilselskabet is built from:

• mobil = mobile / cell phone
• selskab = company

Together, mobilselskab means a mobile phone company or cell phone company.

A useful rule:

• In Danish compounds, the last part is the main noun.
• That last part decides the noun’s gender and usually the basic meaning.

So here:

• the main noun is selskab
• therefore the whole word is neuter: et mobilselskab
• definite form: mobilselskabet

Sender is the present tense form of the verb at sende = to send.

Danish present tense is usually very simple:

• infinitive: at sende
• present: sender

Unlike English, Danish does not change the present-tense verb depending on the subject:

• jeg sender = I send / I am sending
• du sender = you send / you are sending
• mobilselskabet sender = the mobile company sends / is sending

So sender is the normal present-tense form here.

This sentence uses the basic Danish main-clause word order:

• Subject + Verb + Object

So:

• Mobilselskabet = subject
• sender = verb
• regningen = object

That is very similar to English:

• The mobile company sends the bill.

Danish word order becomes more noticeable when something else comes first. For example:

• I dag sender mobilselskabet regningen.
  = Today the mobile company sends the bill.

Then the verb still stays in second position. This is the famous Danish V2 pattern.

Yes. Danish present tense often covers both ideas:

• sends
• is sending

So Mobilselskabet sender regningen could mean either, depending on context.

If the context is general or habitual, it may feel like sends. If the context is happening now, it may feel like is sending.

Danish often lets context do that job instead of changing the verb form the way English does.

A practical learner-friendly pronunciation is roughly:

mo-BEEL-sel-ska-bet

A few notes:

• mobil sounds close to mo-BEEL
• selskab is roughly SEL-skab or SEL-sgab
• the final -et is usually not strongly pronounced like English et; it is often a lighter ending

The main stress is usually earlier in the compound, especially around mobil.

If you are speaking carefully, a good approximation is:

• mo-BEEL-sel-ska-beth
  but with a very light final sound

Native pronunciation can be more reduced than spelling suggests, so listening to recordings is especially helpful with long compounds.

Yes, this is a word many learners find tricky.

A rough approximation is:

RINE-ing-en or RINE-yen

A few important points:

• The gning part often does not sound the way an English speaker expects from the spelling.
• In natural speech, the middle sounds can become softer or more compressed.
• The r and vowels may also sound quite different from English.

If you want a safe learner version, say something close to:

RINE-ing-en

That will usually be understood, even if it is not perfectly native.

Because Danish usually expresses definiteness by adding it to the noun itself.

So instead of:

• the company
• the bill

Danish often uses:

• selskabet
• regningen

This is completely normal.

A separate word like den or det can appear too, but usually in other patterns, especially when an adjective is included:

• regningen = the bill
• den store regning = the big bill

So in your sentence, no extra word for the is needed.

Not normally, if you mean a specific company and a specific bill.

That version sounds incomplete or unnatural in ordinary Danish because both nouns would be indefinite and would also be missing their articles.

Normally you would expect:

• Et mobilselskab sender en regning.
  = A mobile company sends a bill.

or:

• Mobilselskabet sender regningen.
  = The mobile company sends the bill.

So if you want the definite meaning, the sentence you have is the correct natural form.

You change the verb from sender to the past tense sendte:

• Mobilselskabet sendte regningen.

The nouns stay the same unless the meaning changes.

So:

• present: Mobilselskabet sender regningen.
• past: Mobilselskabet sendte regningen.

This is a useful pattern to remember:

• at sende = to send
• sender = sends / is sending
• sendte = sent

In a main question, Danish usually puts the verb first:

• Sender mobilselskabet regningen?

That literally follows the pattern:

• Verb + Subject + Object

So:

• statement: Mobilselskabet sender regningen.
• question: Sender mobilselskabet regningen?

This is very common in Danish yes/no questions.