Ystäväni soitti ovikelloa varovasti, ettei herättäisi lapsia.

My friend rang the doorbell carefully so as not to wake the children.

Breakdown of Ystäväni soitti ovikelloa varovasti, ettei herättäisi lapsia.

• minun: my
• ystävä: the friend
• lapsi: the child
• ettei: so that not
• soittaa: to ring
• ovikello: the doorbell
• varovasti: carefully
• herättää: to wake up

Questions & Answers about Ystäväni soitti ovikelloa varovasti, ettei herättäisi lapsia.

What exactly does Ystäväni mean, and how can it be both singular and plural?

The base word is ystävä = friend.
The ending -ni is a possessive suffix meaning my, so ystäväni literally means my friend.

However, Finnish doesn’t mark plural on the noun here; number is seen from the verb:

• Ystäväni soitti. → My friend called (verb is 3rd singular)
• Ystäväni soittivat. → My friends called (verb is 3rd plural)

In your sentence soitti is singular, so ystäväni = my friend (one person).

Why is it Ystäväni soitti and not something like minun ystävä soitti?

Standard Finnish normally uses either:

• just the possessive suffix: ystäväni soitti
• or genitive + noun + possessive suffix: minun ystäväni soitti

The form minun ystävä soitti (without -ni) is common in colloquial speech, but in written standard language it sounds incomplete or non‑standard.
So for correct written Finnish here you’d say ystäväni soitti or minun ystäväni soitti.

What is the base form of ovikelloa, and what does the word consist of?

The base (dictionary) form is ovikello = doorbell.
It’s a compound of:

• ovi = door
• kello = bell / clock

Ovikelloa is the partitive singular form of ovikello.

Why is it soitti ovikelloa with partitive, and not soitti ovikellon?

With the meaning to ring (a doorbell), soittaa normally takes a partitive object:

• soittaa ovikelloa = to ring the doorbell

This is similar to:

• soittaa viulua = to play the violin (partitive)
• soittaa musiikkia = to play music (partitive)

The partitive here fits an “ongoing / non‑completed” type of action.
Soitti ovikellon would sound odd; it would suggest some kind of complete, one‑off “doorbell event” and is not idiomatic for simply rang the doorbell.

What does varovasti mean and how is it formed?

Varovasti means carefully / cautiously / gently.

It’s formed from the adjective varovainen (careful, cautious) + the adverb ending -sti:

• varovainen → varovasti (careful → carefully)
• similarly: nopea → nopeasti (fast → quickly), hiljainen → hiljaa / hiljaisesti (quiet → quietly)

So soitti ovikelloa varovasti = rang the doorbell carefully / gently.

Why is there a comma before ettei?

In Finnish, a subordinate clause introduced by words like että, koska, vaikka, jos, kun, jotta, ettei is normally separated from the main clause by a comma.

Here:

• Main clause: Ystäväni soitti ovikelloa varovasti
• Subordinate clause: ettei herättäisi lapsia

So the comma is required by standard punctuation rules, even though in English you might sometimes omit it.

What does ettei mean exactly, and how does it differ from että ei or jotta ei?

Ettei is basically a fused form of että ei:

• että = that / so that
• ei = not
  → että ei → ettei

In this context, ettei means so that … not / in order not to:

• … varovasti, ettei herättäisi lapsia.
  = … carefully, so that he wouldn’t wake the children.

You could also say että ei herättäisi lapsia; in everyday language ettei and että ei are practically interchangeable here.
Jotta ei tends to emphasize purpose (in order that … not) a bit more and can sound slightly more formal: jotta ei herättäisi lapsia. All three would be understood.

Why is herättäisi in the conditional mood here?

Herättäisi is the 3rd person singular conditional of herättää (to wake (someone)).
The conditional (-isi-) in this kind of ettei + conditional structure often expresses something undesired or only potential, not an actual fact:

• …, ettei herättäisi lapsia.
  ≈ …, so that he wouldn’t (accidentally) wake the children.

If you used a past indicative, e.g. ettei herättänyt lapsia, it would sound more like a statement about what in fact didn’t happen in the past, rather than about what he was trying to avoid.
So the conditional matches the idea of “preventing a possible outcome” in English.

Who is the subject of herättäisi, and why isn’t it written?

The subject is still ystäväni (my friend).

Finnish usually doesn’t repeat the subject in a subordinate clause if it is the same as in the main clause and the person/number can be seen from the verb:

• Ystäväni soitti ovikelloa varovasti, ettei (hän) herättäisi lapsia.

Adding hän here (… ettei hän herättäisi lapsia) is grammatically fine, but normally unnecessary and slightly more emphatic.
If the subject changed, you would normally show that explicitly:

• Ystäväni soitti ovikelloa varovasti, ettei vauva heräisi.
  My friend rang the doorbell carefully so that the baby wouldn’t wake up.

Why is it lapsia and not lapset?

The base form is lapsi (child).
Lapsia is the partitive plural; lapset is nominative plural (or “total” object form in positive sentences).

Two main reasons for lapsia here:

1. Negative context: ettei = että ei, and with ei the direct object is usually in the partitive:
   • Hän herätti lapset. = He woke the children.
   • Hän ei herättänyt lapsia. = He did not wake (the) children.
2. It feels like an indefinite group: any children / the children there, not a clearly delimited, counted set.

So ettei herättäisi lapsia corresponds well to English so that he wouldn’t wake (any) children.
Ettei herättäisi lapset would be ungrammatical.

Could the word order in the last part be different, for example ettei lapsia herättäisi?

Yes, that word order is possible:

• …, ettei lapsia herättäisi.

The neutral order is … ettei herättäisi lapsia (verb before object).
Putting lapsia earlier (ettei lapsia herättäisi) slightly emphasizes lapsia (“the ones he must not wake are the children”), but the basic meaning stays the same.

Finnish word order is fairly flexible; changes often signal subtle emphasis rather than completely different grammar.

Are there other natural ways to express the same idea in Finnish?

Yes. For example:

• Ystäväni soitti ovikelloa varovasti, jottei herättäisi lapsia.
  (jottei = so that not, a bit more formal than ettei)

Or you can move varovasti:

• Ystäväni soitti varovasti ovikelloa, ettei herättäisi lapsia.

All of these would sound natural; the differences are mostly in style and emphasis, not meaning.

What’s the difference between herättää and herätä?

• herättää is transitive: to wake someone (up)
  
  • Hän herätti lapset. = He woke the children (up).
• herätä is intransitive: to wake up (oneself)
  
  • Lapset heräsivät. = The children woke up.

In your sentence, herättäisi lapsia uses herättää because your friend is the one causing the children to wake up.