Pesen hiukset illalla.

Finnish usually drops subject pronouns because person/number are shown by the verb ending. The ending -n in pesen means “I.”

• Neutral: Pesen hiukset illalla.
• With emphasis or contrast: Minä pesen hiukset illalla.

Finnish often leaves possession unmarked with body parts when the owner is the subject; it’s understood from context. So Pesen hiukset is naturally “I wash my hair.”

Ways to mark “my” if you want:

• Possessive suffix (more formal/explicit): Pesen hiukseni illalla.
• Colloquial pronoun: Mä pesen mun hiukset illalla. (informal) Notes:
• In 3rd person, adding the possessive suffix is common to avoid ambiguity: Hän pesee hiuksensa illalla = “He/She washes his/her hair in the evening.”

Finnish typically refers to head hair in the plural hiukset (“hairs” as a set). The singular hius means “a single strand.” There’s also tukka, a common word meaning “hair (on the head)” as a mass noun:

• Pesen hiukset (standard, very common)
• Pesen tukan (also common; a bit more colloquial/old-fashioned in tone)
• Pesen hiuksen would mean “I wash a single hair (strand),” which is not what you want.

It’s about object case and completeness:

• hiukset (nominative plural) = a “total” object: you wash the whole set of your hair.
• hiuksia (partitive plural) = an “incomplete/indefinite amount” of hair: “some hair,” or hair in general (e.g., as part of a job).

Examples:

• Pesen hiukset illalla. I wash my (whole) hair in the evening.
• Pesen hiuksia työkseni. I wash hair for a living. (hairdresser)

The -t marks the nominative plural. With a plural “total” object in the present tense, Finnish uses nominative plural:

• Pesen hiukset. (plural total object) Compare with a singular total object, which appears with -n:
• Pesen auton. = “I wash the car.”

Illalla = “in/at the evening.” It’s the adessive case (-lla/-llä), commonly used for times of day:

• aamulla (in the morning)
• päivällä (during the day)
• illalla (in the evening)
• yöllä (at night)

For habitual “in the evenings,” use iltaisin. For a specific “this evening,” use tänä iltana (or colloquially tänään illalla).

Yes. Finnish has no separate future tense; the present covers future if the time is clear from context. Pesen hiukset illalla can mean “I will wash my hair this evening.” To be explicit:

• Pesen hiukset tänä iltana. / Pesen hiukset tänään illalla.

Word order is flexible and used for emphasis/focus.

• Neutral: Pesen hiukset illalla.
• Emphasize the time: Illalla pesen hiukset.
• Emphasize the object: Hiukset pesen illalla. All are grammatical; choose based on what you want to highlight.

Use the negative verb and change the object to partitive:

• En pese hiuksia illalla. = “I don’t wash (my) hair in the evening.” If you mark possession:
• En pese hiuksiani illalla.

The verb pestä (“to wash”) has the stem pese- in most forms.

• Present: minä pesen, sinä peset, hän pesee, me pesemme, te pesette, he pesevät
• Past: minä pesin, sinä pesit, hän pesi, me pesimme, te pesitte, he pesivät
• Imperative: pese! (wash!), älä pese! (don’t wash!) Note the 3rd person singular pesee (with a long ee).

Yes. Tukka is another word for head hair. It’s singular and slightly more colloquial or old-fashioned in tone, but very common:

• Pesen tukan illalla.
• Possessive: Pesen tukkani illalla.

Specify the possessor, often with a genitive noun or a 3rd-person possessive suffix:

• Pesen lapsen hiukset. = I wash the child’s hair.
• Pesen hänen hiuksensa. = I wash his/her hair. (the suffix -nsA shows it’s their own hair)