Hänen kasvoillaan näkyy yksi kyynel, mutta kohta hän jo hymyilee.

Kasvoillaan breaks down like this:

• kasvot = face
• kasvoilla = on the face
  
  • this is the adessive case, often used for on
• kasvoillaan = on his/her face
  
  • the ending -an / -än here is a 3rd person possessive suffix

So hänen kasvoillaan means on his/her face.

Because kasvot is a plural-only noun in Finnish. Even when talking about one person’s face, Finnish normally uses the plural form.

So:

• kasvot = face
• kasvoilla = on the face
• kasvoillaan = on his/her face

This is just something you have to learn as part of the word. It is a bit like English nouns such as glasses or scissors, which are plural in form even when they refer to one thing.

It may look redundant from an English point of view, but in standard Finnish this is normal.

• hänen = his/her
• kasvoillaan already contains a 3rd person possessive ending

So hänen kasvoillaan is a standard way to say on his/her face.

Finnish often marks possession this way:

• hänen talonsa = his/her house
• hänen kasvoillaan = on his/her face

So yes, possession is shown twice, but that is normal Finnish grammar, not a mistake.

Because näkyä and nähdä are different verbs.

• nähdä / näkee = to see
• näkyä / näkyy = to be visible, to be seen

In this sentence, no one is actively doing the seeing. The point is that the tear is visible.

So:

• Hän näkee kyyneleen = He/She sees a tear
• Kyynel näkyy = A tear is visible

That is why näkyy is the natural choice here.

Here yksi kyynel is the thing that is visible, so it functions as the subject of the clause, and a singular countable subject like this is normally in the nominative.

So:

• yksi kyynel = one tear

This is similar to other Finnish sentences that introduce something somewhere:

• Pöydällä on kirja = There is a book on the table
• Kasvoillaan näkyy yksi kyynel = One tear is visible on his/her face

If the meaning were more indefinite or plural, another form could appear, for example:

• Kasvoilla näkyy kyyneleitä = There are tears visible on the face

Finnish word order is more flexible than English word order because case endings show the grammatical roles.

Starting with Hänen kasvoillaan puts the location first and sets the scene:

• Hänen kasvoillaan näkyy yksi kyynel
  = On his/her face, one tear is visible

If you said:

• Yksi kyynel näkyy hänen kasvoillaan

that would also be correct, but it puts a little more focus on one tear.

The original sentence sounds natural and descriptive, especially in narration.

Literally, yksi means one.

However, since Finnish has no articles, yksi can sometimes feel a bit like English a/an in translation.

In this sentence, yksi probably does both jobs:

• it tells you the quantity is one
• it also helps introduce a tear into the scene

So the nuance is close to one tear or a single tear, not just a plain article.

Because Finnish often uses the present tense for the near future.

So:

• kohta hän hymyilee = soon he/she will smile or soon he/she is smiling

This is very normal in Finnish, especially with time words like:

• kohta = soon
• huomenna = tomorrow
• ensi viikolla = next week

Finnish does have ways to express the future, but very often the plain present tense is enough.

Jo means already, but here it adds a feeling of quick change or surprisingly fast transition.

So the idea is something like:

• but soon he/she is already smiling
• but before long he/she is smiling already

Without jo, the sentence would still be correct:

• mutta kohta hän hymyilee

With jo, the contrast becomes a little stronger: first there is a tear, but very soon the mood has already changed.

It can mean either one.

Finnish hän is gender-neutral, so:

• hän = he or she
• hänen = his or her

You only know which one is meant from context. Finnish usually does not mark gender in third-person singular pronouns.

Very literal would be something like:

• On his/her face is visible one tear

That sounds unnatural in English, but it helps show the Finnish structure:

• Hänen kasvoillaan = on his/her face
• näkyy = is visible
• yksi kyynel = one tear

A more natural English translation would be:

• One tear is visible on his/her face
• There is a tear visible on his/her face

So the Finnish structure is a little different from English, even though the meaning is straightforward.