Slarva inte med tiden; kom senast kvart i sex om du vill följa med.

Because both verbs are in the imperative, the form used for commands and instructions.

• Slarva inte = Don’t be careless / Don’t mess around
• Kom senast ... = Come no later than ...

In Swedish, just like in English, commands usually leave out the subject:

• Kom! = Come!
• Vänta! = Wait!
• Slarva inte! = Don’t be careless!

The implied subject is you.

Slarva means to be careless, sloppy, or not take something seriously enough.

In this sentence, Slarva inte med tiden means something like:

• Don’t be careless about time
• Don’t mess around with the timing
• Don’t be late / Don’t be casual about punctuality

It does not literally mean slur or anything like that. It is about handling something carelessly.

Here, med tiden means with the time / about the timing.

So:

• slarva med något = be careless with something

Examples:

• slarva med läxorna = be careless with homework
• slarva med pengarna = be careless with money
• slarva med tiden = be careless with time / punctuality

Important: med tiden can also mean over time in other contexts. For example:

• Med tiden blev det bättre = Over time it got better

So the phrase can mean different things depending on context. In your sentence, it clearly means with the time / about punctuality, not over time.

Swedish negative commands are very straightforward:

• imperative + inte

So:

• Kom inte för sent = Don’t come late
• Glöm inte = Don’t forget
• Slarva inte = Don’t be careless

Unlike English, Swedish does not use do not here. You simply use the imperative verb form and add inte.

Senast means at the latest or no later than.

So:

• kom senast kvart i sex = come no later than quarter past five

Other examples:

• Svara senast fredag = Reply by Friday at the latest
• Vi måste gå senast klockan åtta = We must leave no later than eight

It sets a deadline or latest possible time.

Yes, kvart i sex means 5:15.

In Swedish time expressions:

• kvart över fem = quarter past five = 5:15
• kvart i sex = literally quarter to six = 5:15
• halv sex = 5:30
  This one often surprises English speakers, because Swedish counts toward the next hour.

So kvart i sex means there is a quarter of an hour until six.

Yes. Both mean 5:15.

• kvart över fem = quarter past five
• kvart i sex = quarter to six

In practice, Swedish often prefers expressions that count toward the next hour, especially together with forms like halv sex for 5:30. But both versions are understandable.

Literally, yes, it is made up of:

• följa = follow
• med = with

But as a phrase, följa med usually means:

• come along
• join
• go with someone

So in this sentence:

• om du vill följa med = if you want to come along

This is a very common Swedish particle verb, and its meaning is more natural than the literal follow with.

Because after the modal verb vill (want to), Swedish normally uses the infinitive without att.

So:

• du vill följa med = you want to come along
• jag kan simma = I can swim
• hon måste gå = she must leave

This is similar to English, where we also do not say you want to to come along.
A simpler rule: after modal verbs like kan, ska, vill, måste, får, Swedish usually does not use att.

Because this is a normal subordinate clause introduced by om (if).

The structure is:

• om
  • subject + verb + infinitive phrase

So:

• om du vill följa med = if you want to come along

This is standard Swedish subordinate-clause word order. Also notice that there is no extra inversion after om. You simply get:

• om du kommer = if you come
• om han vill hjälpa = if he wants to help

The semicolon links two closely related independent clauses:

• Slarva inte med tiden
• kom senast kvart i sex om du vill följa med

It works a bit like a strong comma or a softer full stop. The meaning is:

• Don’t be careless about the time; come no later than 5:15 if you want to come along.

In ordinary writing, you might also see:

• a full stop: Slarva inte med tiden. Kom senast ...
• sometimes even a comma in less formal writing

So the semicolon is a punctuation choice, not a special grammar feature.

With kom, it means come / arrive by then.

So the sentence is telling the person to be there no later than 5:15.

If Swedish wanted to talk about leaving by a certain time, it would more naturally use another verb, for example:

• Gå senast kvart i sex = Leave no later than 5:15
• Åk senast kvart i sex = Travel/go no later than 5:15

So kom here clearly points to arriving or showing up.