Rengas oli tyhjä, kun lähdin kotoa.

In Finnish, tyhjä literally means empty, but with a tire it naturally means flat—in other words, the tire has no air in it.

So:

• rengas on tyhjä = the tire is empty of air = the tire is flat

This is a normal and idiomatic way to say it in Finnish.

Oli is the past tense of olla (to be).

• on = is
• oli = was

Because the sentence talks about a past situation, Finnish uses the past tense:

• Rengas oli tyhjä = The tire was flat

Also, oli is the third person singular form, because rengas is singular.

Finnish often leaves out subject pronouns when they are already clear from the verb ending.

• lähdin = I left
• lähdit = you left
• lähti = he/she/it left

So kun lähdin kotoa already means when I left home, even without minä.

You could say kun minä lähdin kotoa, but the minä is usually unnecessary unless you want extra emphasis.

Lähdin is the first person singular past tense of lähteä, which means to leave or to depart.

Breakdown:

• dictionary form: lähteä
• past stem: lähd-
• first person singular ending: -in

So:

• lähdin = I left

This is a very common Finnish past-tense pattern.

Because Finnish uses different forms depending on location or movement.

With koti (home), the common location forms are:

• kotona = at home
• kotoa = from home
• kotiin = to home / homeward

Since the sentence says left home, Finnish uses the from form:

• lähdin kotoa = I left home

This is something you mostly just learn as a set with koti, because home behaves a little specially in Finnish.

Here tyhjä is a predicate adjective after the verb olla.

In a basic sentence like this, the predicate adjective is usually in the nominative and agrees with the subject in number:

• rengas = singular
• tyhjä = singular nominative

So:

• Rengas oli tyhjä.

If the subject were plural, the adjective would also be plural:

• Renkaat olivat tyhjät. = The tires were flat.

Kun here means when.

It introduces a subordinate clause that gives the time of the main action or state:

• Rengas oli tyhjä = the tire was flat
• kun lähdin kotoa = when I left home

So the whole sentence means that the tire was flat at the time I left home.

In other contexts, kun can also mean as, once, or even because depending on usage, but here the meaning is clearly when.

Yes. Finnish word order is fairly flexible.

You can also say:

• Kun lähdin kotoa, rengas oli tyhjä.

This means the same thing: When I left home, the tire was flat.

The difference is mainly emphasis:

• Rengas oli tyhjä, kun lähdin kotoa.
  starts with the fact that the tire was flat
• Kun lähdin kotoa, rengas oli tyhjä.
  starts with the time frame

Both are natural.

Because Finnish normally uses a comma between a main clause and a subordinate clause.

So:

• Rengas oli tyhjä, kun lähdin kotoa.

If the subordinate clause comes first, the comma still appears, but after that clause:

• Kun lähdin kotoa, rengas oli tyhjä.

This comma usage is standard Finnish punctuation.

Finnish does not have articles like a, an, or the.

So rengas can mean:

• a tire
• the tire

The exact meaning depends on context.

In this sentence, English will usually need the tire or sometimes a tire, but Finnish simply says rengas.

Grammatically, rengas is just singular: tire.

Finnish does not automatically mark whether it is a tire or the tire, so context decides that. In a real-life situation, it often means some relevant tire on a vehicle—usually understood from the situation.

If you wanted to talk about more than one tire, you would use the plural:

• renkaat olivat tyhjät = the tires were flat

So in your sentence, only one tire is being referred to.