The Active Temporal Construction
To express that two actions happen simultaneously (like "while waiting" or "when the tram arrives"), Finnish often uses the active temporal construction instead of a kun (when) clause.
Form the 2nd infinitive inessive by taking the dictionary form, replacing the final -a or -ä with -e, and adding the inessive ending -ssa or -ssä.
| When the tram comes, the passengers must look for seats. | Raitiovaunun tullessa matkustajien pitää etsiä istumapaikkoja. |
| When the subway stops, we hear an important announcement. | Metron pysähtyessä kuulemme tärkeän kuulutuksen. |
Different Subjects vs. Same Subject
How you indicate the subject of the temporal action depends on whether it matches the main subject of the sentence.
Different subjects: If the doer of the "while/when" action is different from the main subject, put the temporal subject in the genitive case (ending in -n or plural -en / -ien).
| When the inspector checks travel cards, there is one angry passenger in the train. | Tarkastajan tarkistaessa matkakortteja junassa on yksi vihainen matkustaja. |
| When the passengers step into the tram, it is raining outside. | Matkustajien astuessa raitiovaunuun ulkona sataa. |
Same subject: If the same person performs both actions (e.g., "When I stand, I listen"), do not use a genitive pronoun. Instead, attach a possessive suffix directly to the -essa/-essä ending.
| When standing on the platform, I listened to announcements. | Laiturilla seisoessani kuuntelin kuulutuksia. |
| When you go into the subway, you should validate the travel card. | Metroon mennessäsi sinun kannattaa leimata matkakortti. |
Stem Changes for -ea / -eä Verbs
If a verb's dictionary form ends in -ea or -eä (like lukea, to read, or lähteä, to leave), the e shifts to an i before adding the -essa / -essä ending (e.g., lukea becomes lukiessa, lähteä becomes lähtiessä).
| When reading a book on the subway, I need a seat. | Lukiessani kirjaa metrossa tarvitsen istumapaikan. |
| When the commuter train leaves, there is still an empty seat here. | Lähijunan lähtiessä täällä on vielä tyhjä istumapaikka. |