Suljen hanan, koska vesi on liian kylmää.

Suljen is the 1st person singular present tense of the verb sulkea (to close).
Finnish often uses the present tense for actions happening right now or in the near future, so Suljen hanan means I’m closing the tap / I’ll close the tap depending on context.

The stem changes slightly in conjugation:

• Infinitive: sulkea
• Present stem: sulje-
• 1st person ending: -n So: sulje + n → suljen.
  This is a common pattern for verbs ending in -ea/-eä.

Hana means a tap/faucet. It’s the general everyday word in Finnish for the thing you turn to control water.

Because hana is the object of the verb suljen (I close X), and Finnish marks many total/complete objects with the accusative, which for singular nouns often looks like the genitive -n.
So hanan here is essentially “the tap (as a complete object being closed).”

Form-wise, hanan looks like the genitive singular (ending -n). In many sentences, that same -n form is also used for the accusative singular (the “total object”).
In practice for learners, it’s easiest to think:

• Suljen hanan = “I close the tap (completely)” → total object → -n form

Yes, suljen hanaa (partitive object) is possible, but it changes the nuance:

• Suljen hanan = I close the tap (a complete, finished action / the tap is shut)
• Suljen hanaa = I’m closing the tap (process/ongoing), or closing it “somewhat,” or in contexts where the completion isn’t emphasized
  For a normal, complete “turn it off,” hanan is the most typical.

Koska means because and introduces a subordinate clause.
Finnish word order is fairly flexible, but a basic structure remains:

• Main clause: Suljen hanan
• Because-clause: koska vesi on liian kylmää
  Unlike some languages, Finnish doesn’t force a special “verb-last” order here; vesi on is perfectly normal.

Finnish doesn’t use a dummy subject like English it in weather/condition statements. You simply state the subject:

• vesi = the water
• on = is
  So vesi on... = the water is...

After olla (to be), Finnish often uses the partitive for adjectives describing:

• an unbounded quality (a general property, not a fixed amount), or
• a state/condition in a general sense

So vesi on kylmää is the natural way to say the water is cold.
The partitive singular of kylmä is kylmää.

Because vesi is the subject here, and subjects are typically in the nominative (basic dictionary form): vesi.
The predicate adjective (what the subject “is like”) can be partitive: kylmää.

Liian means too. It modifies an adjective or adverb:

• liian kylmää = too cold
  It usually comes directly before the word it modifies, as it does here.

Finnish negation uses the negative verb en with the main verb in a special form:

• En sulje hanan, koska vesi on liian kylmää. = I’m not closing the tap, because the water is too cold.
  (Here sulje is the connegative form used with en.)

A few common points for English speakers:

• Suljen: the lj is like a “soft” l + y glide for many speakers (varies by dialect), roughly sul-yen.
• hanan: stress on first syllable: HA-nan.
• koska: KOS-ka.
• liian: long ii: lii-an (two syllables).
• kylmää: front vowel y (like German ü), and long ää at the end.