Käyn usein kylvyssä.

Käyn is the 1st person singular present tense of käydä.

• käydä = to go, to visit, to go and do something
• käyn = I go / I visit / I go and do

In Käyn usein kylvyssä, the verb is part of the expression käydä kylvyssä, which idiomatically means to have / take a bath.

Finnish often leaves the subject pronoun out when it is already clear from the verb ending.

• käyn already means I go
• so Käyn usein kylvyssä is a complete sentence

You could also say Minä käyn usein kylvyssä, but that adds emphasis, contrast, or personal focus.

Because käydä is very common in Finnish for going somewhere to do an activity.

So:

• käydä kylvyssä = to have a bath
• käydä saunassa = to go to the sauna / have a sauna
• käydä suihkussa = to take a shower

If you used mennä, the focus would be more on literal movement:

• Menen kylpyyn = I go into the bath / I am going to the bath

That is different from the more idiomatic activity expression käydä kylvyssä.

Kylvyssä is the inessive case of kylpy.

The inessive ending is:

• -ssa or -ssä
• it usually means in or inside

So literally:

• kylvyssä = in a bath

But in this sentence it works idiomatically, so the whole expression means having / taking a bath, not just physically being located inside one.

The ending is -ssä rather than -ssa because of vowel harmony: the word has front vowels like ä and y, so it takes the front-vowel version -ssä.

This is a normal Finnish stem change in inflection.

The basic form is:

• kylpy

But in many other forms the stem becomes:

• kylvy-

For example:

• kylpy
• kylvyn
• kylvyssä

So the p changes to v in these forms. This is part of the noun’s inflection pattern, and it is something learners usually just get used to by seeing more examples.

Yes, very common.

Finnish often uses käydä with a place or activity in the inessive case to mean go and do something there or go there as an activity.

Some common examples:

• käydä töissä = to go to work / be working
• käydä koulussa = to go to school
• käydä saunassa = to go to the sauna
• käydä suihkussa = to take a shower
• käydä uimassa is slightly different in form, but similar in idea: going to do an activity

So käydä kylvyssä fits a very common Finnish structure.

That is a normal, neutral word order in Finnish.

• Käyn usein kylvyssä = neutral, natural
• Usein käyn kylvyssä = possible, but gives more emphasis to often
• Kylvyssä käyn usein = possible, but more marked or stylistic

Finnish word order is more flexible than English, but the version in your sentence is the most straightforward everyday order.

In most ordinary contexts, the most natural English meaning is:

• I often take a bath
• or I often have a bath

Depending on context, it could also feel a bit like I often go bathing, but for everyday translation, I often take a bath is the best choice.

Finnish has no articles, so it does not directly mark a or the.

That means kylvyssä does not itself tell you:

• in a bath
• in the bath

English has to choose the most natural article from context. In this sentence, English normally says take a bath.

Yes, that is possible, and it also means I often take a bath.

But Käyn usein kylvyssä is a very natural Finnish way to express the idea. The two are close in meaning, but käydä kylvyssä feels especially idiomatic as an everyday activity expression, similar to käydä suihkussa or käydä saunassa.