Säästän rahaa keväällä vuokratakseni mökin heinäkuussa, kun kesäloma alkaa.

In Finnish, the subject pronoun is often left out when it is clear from the verb ending.

• säästän = I save / I am saving
• The ending -n already shows first person singular

So Minä säästän is possible, but it usually adds emphasis. In a neutral sentence, just Säästän sounds natural.

Rahaa is in the partitive case, which is very common after verbs like säästää when you mean money in a general or unspecified amount.

Here, Säästän rahaa means something like:

• I am saving money
• not I am saving the money
• and not I save one complete, specific amount

If you said säästän rahat, that would mean something more like I save the money or I keep the money, referring to a specific whole amount.

Keväällä means in spring / during the spring.

It is the word kevät in the adessive case. With seasons, Finnish often uses this case for time expressions:

• keväällä = in spring
• kesällä = in summer
• syksyllä = in autumn
• talvella = in winter

So although -llä often means on or at, in time expressions like this it is best understood as during / in.

Vuokratakseni expresses purpose: in order to rent or so that I can rent.

It comes from the verb vuokrata and a special infinitive construction used to show purpose. In this sentence:

• Säästän rahaa ... vuokratakseni mökin
• I save money ... in order to rent a cottage

This is a very common written and standard way to express purpose in Finnish.

The -ni shows that the person doing the purpose action is I.

So:

• vuokratakseni = for me to rent / in order for me to rent
• vuokratakseen = for him/her to rent
• vuokrataksemme = for us to rent

In this sentence, the subject of säästän and the understood subject of vuokratakseni are the same person, so -ni is used.

Yes. That would also be correct.

• vuokratakseni mökin = in order to rent a cottage
• jotta voin vuokrata mökin = so that I can rent a cottage

The version with vuokratakseni is more compact. The jotta voin... version is a little more explicit and may feel easier to learners at first.

Mökin is the total object form here.

With vuokrata, if you mean renting one whole cottage, Finnish uses the total object:

• vuokrata mökki is the dictionary form
• in an actual sentence, the object often appears as mökin

So vuokratakseni mökin means renting a complete, specific cottage, not just doing some vague or partial renting activity.

A useful contrast:

• vuokrata mökin = rent a whole cottage
• vuokrata mökkiä = rent a cottage, but with a more incomplete/ongoing/open-ended feel

Heinäkuussa means in July.

Months in Finnish are usually put in the inessive case for time expressions:

• tammikuussa = in January
• kesäkuussa = in June
• heinäkuussa = in July

The ending -ssa / -ssä often means in, and with months that is exactly how it works.

Here kun means when.

So:

• kun kesäloma alkaa = when the summer holiday begins

Be careful, because kun can also mean because in some contexts. In this sentence, the time meaning when is the natural one.

Finnish often uses the present tense for future events when the time is already clear from context.

Here, the future meaning is obvious because of:

• heinäkuussa = in July
• the whole sentence describes a planned future situation

So kun kesäloma alkaa naturally means when the summer holiday begins / will begin.

Finnish does not usually need a separate future tense the way English often does.

Finnish has no articles, so there is no direct equivalent of a or the.

That means kesäloma can mean:

• summer holiday
• the summer holiday
• sometimes even a summer holiday

The exact meaning depends on context. In this sentence, English naturally uses the: when the summer holiday begins.

Finnish word order is fairly flexible, although this version is natural and clear.

This sentence has a neutral flow:

• Säästän rahaa = main action
• keväällä = when
• vuokratakseni mökin = purpose
• heinäkuussa = when the renting happens
• kun kesäloma alkaa = extra time clause

You could move parts around for emphasis. For example:

• Keväällä säästän rahaa vuokratakseni mökin heinäkuussa, kun kesäloma alkaa.

That still works, but it emphasizes keväällä more strongly.