Learn Hardcore Finnish: Jätän puhelimen kotiin viikonlopun ajaksi. - I leave the phone at home for the weekend.

puhelin = nominative (dictionary form)
puhelimen = genitive singular; for a total/complete object with a personal subject, it doubles as the accusative

In this sentence:

Jätän puhelimen = I (will) leave the phone (a specific, whole thing)

Using plain puhelin here (Jätän puhelin) would be ungrammatical. With a personal subject in the present tense and a total object, Finnish chooses the -n form for the object (here: puhelimen).

Why is it kotiin and not kotona, koti or something else?

All of these are related to koti (home), but they express different spatial relations:

kotiin = to home, into home (direction towards home)
kotona = at home (location, being at home)
kotoa = from home (movement away from home)
plain koti is the base form and usually needs a case ending in real sentences

In Jätän puhelimen kotiin, the idea is:

I leave the phone *to home → I leave the phone *at home (i.e. I don’t take it with me; it stays there)

You need kotiin because the movement is conceptually towards home: the phone’s final location is home.

What does viikonlopun ajaksi literally mean?

Literally, viikonlopun ajaksi breaks down as:

viikonloppu = weekend
viikonlopun = genitive: of the weekend
aika = time
ajaksi = translative form of aika → for a time / for a period

So viikonlopun ajaksi literally is:

for the time of the weekend → more naturally: for the weekend / for the duration of the weekend.

It emphasizes the duration: the phone will remain at home for that whole period.

Could I just say viikonlopuksi instead of viikonlopun ajaksi?

Yes, you can.

Jätän puhelimen kotiin viikonlopun ajaksi.
Jätän puhelimen kotiin viikonlopuksi.

Both mean roughly I’ll leave my phone at home for the weekend.

Differences:

viikonlopun ajaksi
- slightly more explicit and “full”: for the (whole) time of the weekend
viikonlopuksi (translative of viikonloppu)
- very natural, compact, and common: for the weekend

In normal speech, viikonlopuksi is very typical; viikonlopun ajaksi sounds a bit more explicit or careful, but not formal or strange.

Is it possible to drop ajaksi and just say Jätän puhelimen kotiin viikonlopun?

No, that sounds wrong or at least very unnatural.

You need some way to show the “for (a duration)” meaning. In practice, you almost always use:

viikonlopuksi, or
viikonlopun ajaksi

Plain viikonlopun after this verb, without ajaksi or the -ksi ending, does not by itself mean for the weekend.

Can I change the word order, like Jätän viikonlopun ajaksi puhelimen kotiin?

Yes, Finnish word order is quite flexible, and your example is grammatically fine:

Jätän viikonlopun ajaksi puhelimen kotiin.
Jätän puhelimen kotiin viikonlopun ajaksi.

Both are correct. The default-sounding version is the original, but moving parts around usually changes emphasis, not correctness.

Rough guide:

Early in the sentence = more topical or emphasized.
So putting viikonlopun ajaksi earlier gives a bit more weight to the duration, whereas the original flows more like neutral information in English.

How would I say “for the whole weekend” or “only for the weekend”?

You can modify viikonlopuksi / viikonlopun ajaksi with koko (whole) or vain (only).

Examples:

Jätän puhelimen kotiin koko viikonlopuksi.
→ I’ll leave my phone at home for the whole weekend.
Jätän puhelimen kotiin vain viikonlopuksi.
→ I’ll leave my phone at home only for the weekend.

Same works with ajaksi:

Jätän puhelimen kotiin koko viikonlopun ajaksi.
Jätän puhelimen kotiin vain viikonlopun ajaksi.

Why does Finnish use the present tense here, when in English we’d usually say “I will leave”?

Finnish often uses the present tense to talk about:

planned future actions
scheduled events
near-future intentions

So:

Jätän puhelimen kotiin viikonlopun ajaksi.
→ natural translation: I will leave my phone at home for the weekend.

You could theoretically emphasize future with something like Aion jättää (I intend to leave), but it’s not necessary. Plain present (jätän) is the normal way to express a planned action in the near future.

Why doesn’t the sentence say “my phone”? How do I show possession?

Finnish usually omits possessive words like “my” when it’s obvious from context.

In Jätän puhelimen kotiin, it’s naturally understood to be your phone, unless the context says otherwise.

If you want to be explicit, you can use a possessive suffix:

Jätän puhelimeni kotiin viikonlopun ajaksi.
→ literally I leave my-phone home for the weekend.

This is more emphatic that it’s specifically my phone, but it can also sound a bit more formal or “written” depending on context. In everyday speech, simply puhelimen is usually enough.

What is the role of ajaksi grammatically, and can I use it with other time expressions?

Ajaksi is the translative case of aika (time). In this kind of structure (genitive noun + ajaksi), it functions like a postposition meaning “for the duration of”.

Pattern:

[time in genitive] + ajaksi = for the time of X / for X (duration)

Examples:

kahden tunnin ajaksi = for two hours
lomani ajaksi = for the duration of my vacation
talven ajaksi = for the winter

So viikonlopun ajaksi fits the same pattern: for the time of the weekend → for the weekend.

Is there a difference in meaning between Jätän puhelimen kotiin viikonlopuksi and Jätän puhelimen kotiin viikonloppuna?

Yes, there is a nuance:

viikonlopuksi (translative “for the weekend”)
→ emphasizes duration: the phone will stay at home for the whole weekend.
viikonloppuna (inessive “during the weekend”)
→ focuses on when something happens: during the weekend.

So:

Jätän puhelimen kotiin viikonlopuksi.
→ I’ll leave my phone at home for the weekend (it will remain there).
Jätän puhelimen kotiin viikonloppuna.
→ I (will) leave my phone at home during the weekend (the act of leaving takes place at some point in the weekend; the duration afterwards is less strongly expressed).

For the English idea “I’m not taking my phone with me for the whole weekend,” viikonlopuksi / viikonlopun ajaksi is the more natural choice.