Learn Hardcore Finnish: He lähettävät toisilleen viestejä iltaisin. - They send messages to each other in the evenings.

Finnish doesn’t mark gender at all, so hän can mean he or she depending on context. In this sentence, he tells you we’re talking about more than one person.

How is lähettävät formed, and why does it end in -vät?

The basic dictionary form is lähettää (to send). The stem is lähettä‑.

To make the present tense for he (they), you add the 3rd person plural ending ‑vät (or ‑vat, depending on vowel harmony):

lähettä‑
- vät → lähettävät

Why ‑vät and not ‑vat?
Because the word has front vowels (ä), the ending also uses the front-vowel version ‑vät. With back vowels (a, o, u) you’d see ‑vat instead, e.g. puhuvat (they speak).

So lähettävät literally means “they send”.

Could you leave out he in this sentence?

Yes, you can, and it’s actually very common in Finnish:

Lähettävät toisilleen viestejä iltaisin.

The verb form lähettävät already shows that the subject is 3rd person plural (they), so the pronoun he is not grammatically necessary.

You include he if you want to:

start a new topic (make it clear who “they” are), or
emphasize the subject (e.g. contrast with someone else).

Otherwise, dropping it is natural and normal.

What exactly does toisilleen mean, and how is it built?

Toisilleen means “to each other”.

It comes from toinen (other, another, one of two), and is built like this:

toinen → toisille = to the others (allative plural: “to others”)
toisille
- ‑en (3rd person possessive suffix) → toisilleen

The possessive suffix ‑en in this word gives it the reciprocal meaning “to each other”, not just “to some other people”.

So he lähettävät toisilleen = they send to each other (mutually).

Why is it toisilleen and not toisille or toisiaan here?

All three forms exist, but they mean slightly different things.

toisille
- Allative plural without possessive suffix
- Means: “to others” (some other people, not necessarily each other)
- He lähettävät viestejä toisille. = They send messages to others.
toisilleen
- Allative plural + 3rd person possessive suffix
- Means: “to each other” (reciprocal)
- He lähettävät toisilleen viestejä. = They send messages to each other.
toisiaan
- Partitive with a possessive suffix: reciprocal direct object
- Used when “each other” is the object, not a recipient
- He rakastavat toisiaan. = They love each other.
- He näkevät toisiaan. = They see each other.

With lähettää (to send), the recipient is normally in the allative case (lähettää jollekulle = send to someone), so toisilleen is the natural form when the recipients are “each other.”

Why is viestejä in the partitive plural instead of viestit?

Viestejä is the partitive plural of viesti (message).

Finnish uses the partitive for objects when the amount is:

indefinite / unspecified, or
the action is ongoing, repeated, or not seen as a complete total.

Here, viestejä suggests:

an unspecified number of messages
a general, repeated activity (they habitually send some messages in the evenings)

If you say viestit (nominative plural), you usually mean a specific, whole set of messages that this action covers, e.g. “the messages” (those particular ones), rather than “messages (in general).”

So viestejä fits better for a habitual activity with no specific count: they send messages (some messages, messages in general).

Could you use viestit instead of viestejä, and how would the meaning change?

You could say He lähettävät toisilleen viestit iltaisin, but the nuance changes:

viestejä:
- “messages” in general; indefinite quantity
- focus on the activity of sending some messages (habitual)
viestit:
- “the messages”; a specific, definite set
- implies there are particular messages that get sent (for example, some fixed standard messages, or all of the messages in some known set)

In everyday speech about a general habit, viestejä is by far the more natural choice.

What form is iltaisin, and why not illalla?

Iltaisin is an adverbial form that means “in the evenings / in the evening (as a habit)”.

It’s historically related to the essive plural of ilta (evening) and is part of a common pattern used for habitual or repeated time:

aamu → aamuisin (in the mornings)
päivä → päivisin (in the daytime, during days)
yö → öisin (at nights)
ilta → iltaisin (in the evenings)

Illalla is the adessive singular and typically refers to one specific evening or a single time frame:

Illalla: (on) the evening (today / on that day)
Iltaisin: in the evenings generally, usually in the evenings (habit)

Since the sentence describes a repeated habit, iltaisin is the natural form.

Can you change the word order, for example to Iltaisin he lähettävät toisilleen viestejä?

Yes. Finnish word order is fairly flexible, and you can move elements around to change emphasis.

Some natural options:

He lähettävät toisilleen viestejä iltaisin.
Neutral: they send messages to each other in the evenings.
Iltaisin he lähettävät toisilleen viestejä.
Emphasizes when they do it (in the evenings is the important part).
He iltaisin lähettävät toisilleen viestejä.
Also possible, but putting iltaisin right after the subject feels a bit more marked; it still adds emphasis to the time.

As long as the verb agrees with the subject and the cases are correct, different word orders are mostly about focus and style, not basic grammar correctness.

Why doesn’t viestejä have a possessive ending, like viestejään?

The form viestejään (partitive plural + possessive suffix) would mean “their (own) messages”, explicitly marking possession.

He lähettävät toisilleen viestejä.
= They send messages to each other (no explicit emphasis on whose messages they are).
He lähettävät toisilleen viestejään.
= They send their own messages to each other (more clearly “their messages,” not somebody else’s).

In many everyday contexts, Finnish omits the possessive suffix when it’s obvious from context whose things are involved. Since it’s natural to assume people send their own messages to each other, viestejä is enough, and viestejään can sound more explicit or slightly heavier in style.