De trapper zit los, dus ik fiets vandaag langzamer.

Yes. De trapper means the pedal, especially on a bicycle.

A few useful notes:

• trapper is a very common everyday word for a bike pedal
• pedaal also exists and is a bit more general or technical
• in this sentence, de trapper is singular, so it means one pedal is loose

So the sentence suggests that one of the bike’s pedals is not firmly attached.

Because trapper is a de-word in Dutch.

In Dutch, singular nouns usually take either:

• de
• het

You mostly have to learn which article belongs to each noun. For this word:

• de trapper
• plural: de trappers

So this is just part of the noun’s gender/class.

This is a very common Dutch pattern.

Zitten does not only mean to sit. It is also often used for the way something is attached, positioned, or fixed.

So:

• De trapper zit los = The pedal is loose / has come loose
• De knop zit vast = The button is stuck / firmly attached
• Het label zit scheef = The label is crooked

In English, sit usually would not be used this way, but in Dutch it is very normal.

Here los means loose, not firmly attached, or coming away from where it should be fixed.

So De trapper zit los means the pedal is wobbling, not secure, or partly detached.

In other contexts, los can also mean things like:

• free
• detached
• separate
• not connected

But in this sentence, loose is the right idea.

Here fiets is a verb.

The verb is fietsen, which means to cycle / to ride a bike.

Conjugation:

• ik fiets = I cycle
• jij fietst = you cycle
• wij fietsen = we cycle

So in ik fiets vandaag langzamer, fiets means cycle / ride my bike.

This can confuse learners because fiets is also the noun bike:

• de fiets = the bike

So Dutch has:

• de fiets = the bicycle
• ik fiets = I cycle

Here langzamer means more slowly.

It is the comparative form of langzaam:

• langzaam = slow / slowly
• langzamer = slower / more slowly

Dutch often uses the same form for adjectives and adverbs, so:

• een langzame fiets = a slow bike
• ik fiets langzaam = I cycle slowly
• ik fiets langzamer = I cycle more slowly

In this sentence, langzamer is functioning like an adverb because it describes how the person cycles.

Because the comparison is implied.

Langzamer often appears without dan when the speaker means something like:

• more slowly than usual
• more slowly than normal
• more slowly than I otherwise would

So:

• Ik fiets vandaag langzamer.
  = I’m cycling more slowly today.

If you want to say the comparison explicitly, you can add it:

• Ik fiets vandaag langzamer dan normaal.
• Ik fiets vandaag langzamer dan gisteren.

Yes, vandaag can move.

In the given sentence:

• ik fiets vandaag langzamer

vandaag sits in the middle of the clause, which is very natural.

You can also say:

• Vandaag fiets ik langzamer.
• Ik fiets langzamer vandaag. — possible, but less neutral in many contexts

The most neutral options are usually:

• Ik fiets vandaag langzamer
• Vandaag fiets ik langzamer

The placement changes emphasis slightly:

• Vandaag fiets ik langzamer puts more focus on today
• Ik fiets vandaag langzamer sounds more neutral

This is a great question, because it touches on a real Dutch word-order issue.

In careful standard Dutch, if dus comes at the start of the second main clause, you will often see inversion:

• De trapper zit los, dus fiets ik vandaag langzamer.

That is because dus is traditionally treated more like a conjunctive adverb than a simple coordinating conjunction.

However, in everyday speech and informal writing, many speakers also say:

• De trapper zit los, dus ik fiets vandaag langzamer.

So the version you were given is very understandable and quite common informally.

A useful learner rule:

• for formal/careful Dutch, dus fiets ik ... is the safer choice
• for informal everyday Dutch, dus ik fiets ... is also common

Because the sentence contains two main clauses:

• De trapper zit los
• (dus) ik fiets vandaag langzamer

A comma is normally used when these kinds of clauses are linked in this way.

So the comma helps show the pause and the logical connection:

• first clause: the problem
• second clause: the result

Yes, singular is completely normal.

A bike has two pedals, but if only one of them is loose, Dutch can naturally say:

• De trapper zit los.

That usually means one pedal is loose, often the one being noticed right now.

If both were loose, you could say:

• De trappers zitten los.

So singular vs. plural works exactly as you would expect.