Der Download dauert heute lange, weil das Internet langsam ist.

In standard German, Download is usually masculine: der Download.

So in the nominative case, you say:

• der Download
• der Download dauert lange

German loanwords do not automatically become neuter. Their gender has to be learned case by case. For technology words, there is often no obvious rule, so learners usually just memorize the article together with the noun.

A good habit is to learn new nouns as a full unit:

• der Download
• das Internet

dauert is from the verb dauern, which means to last or to take (time).

So:

• Der Download dauert lange = The download is taking a long time / lasts a long time

This is a very common German structure:

• Es dauert lange. = It takes a long time.
• Die Reise dauert zwei Stunden. = The trip takes two hours.

So German uses dauern where English often uses take.

Here, lange is being used adverbially, describing how long the download takes.

• Der Download dauert lange. = The download takes a long time.

This is the usual form after dauern.

Compare:

• Der Film ist lang. = The film is long.
  Here lang is an adjective describing the noun via sein.
• Der Film dauert lange. = The film lasts a long time.
  Here lange works adverbially with dauern.

For learners, the safest pattern is:

• etwas dauert lange

German main clauses follow the verb-second rule: the finite verb must be in the second position.

Here the structure is:

• Der Download = position 1
• dauert = position 2
• heute lange = the rest

So:

• Der Download dauert heute lange

This is normal German word order.

You can also move heute to the front for emphasis:

• Heute dauert der Download lange, weil das Internet langsam ist.

That still works because dauert remains in second position.

German adverbs often follow a general tendency where time comes earlier than things like manner or degree.

So:

• heute = time
• lange = duration/extent

That makes heute lange sound natural:

• Der Download dauert heute lange.

If you said lange heute, it would sound unnatural in this sentence.

A simple learner rule is:

• time expressions often come before words like lange, schnell, gut, etc.

Because weil introduces a subordinate clause, and in German subordinate clauses the finite verb usually goes to the end.

So:

• Main clause: Das Internet ist langsam.
• After weil: weil das Internet langsam ist

This is one of the most important word-order rules in German.

Compare:

• Das Internet ist langsam.
• ..., weil das Internet langsam ist.

The subject and other information come first, and the conjugated verb moves to the end.

Yes, but the word order changes.

With weil, the verb goes to the end:

• Der Download dauert heute lange, weil das Internet langsam ist.

With denn, the clause keeps normal main-clause word order:

• Der Download dauert heute lange, denn das Internet ist langsam.

Both mean because, but:

• weil is very common in everyday German
• denn sounds a bit more formal or written in some contexts

So both are correct, but weil is often the more common spoken choice.

Because Internet is a neuter noun in German: das Internet.

So you say:

• das Internet
• das Internet ist langsam

Like any German noun, its gender must be learned together with the word. There is no perfect logic here from an English speaker’s point of view, so it is best to memorize:

• das Internet
• der Download

In German, all nouns are capitalized, not just proper names.

So:

• der Download
• das Internet

This is a basic spelling rule in German. Even ordinary nouns like these begin with a capital letter.

Compare:

• Der Mann liest ein Buch.
• Das Internet ist langsam.

The nouns Mann, Buch, and Internet are all capitalized.

Grammatically, langsam describes das Internet:

• das Internet langsam ist = the internet is slow

That is the direct meaning of the clause.

In natural English, we might think the download is slow, but German here expresses the cause differently:

• The download takes a long time because the internet is slow.

So:

• lange describes how long the download takes
• langsam describes the speed of the internet

In standard German, the usual form is der Download.

You should use:

• der Download

You may occasionally hear variation in different regions or from different speakers, especially with modern English loanwords, but for learners the safest and most standard choice is definitely der Download.

Yes. If you remove heute, the sentence still works perfectly:

• Der Download dauert lange, weil das Internet langsam ist.

Adding heute just narrows it to today. It suggests that today, in particular, the download is taking a long time.

So:

• without heute = general statement
• with heute = statement about today

Yes, it is natural and correct.

A native speaker might also say things like:

• Der Download dauert heute ewig, weil das Internet so langsam ist.
• Der Download ist heute langsam, weil das Internet langsam ist.
• Heute dauert der Download lange, weil das Internet langsam ist.

But your original sentence is completely normal and idiomatic:

• Der Download dauert heute lange, weil das Internet langsam ist.

It is especially good German because it uses:

• a natural verb: dauern
• correct adverb placement: heute lange
• correct subordinate clause word order after weil