Breakdown of Metroon mennessäsi sinun kannattaa leimata matkakortti.
sinä
you
mennä
to go
kannattaa
to be profitable / to be worth it
metro
subway
Questions & Answers about Metroon mennessäsi sinun kannattaa leimata matkakortti.
What makes up the word mennessäsi in this sentence?
It uses the active temporal construction (temporaalirakenne), which replaces a 'when' clause. It is built from the second infinitive of mennä in the inessive case (mennessä) plus the possessive suffix -si to indicate 'you' are the one going. It means exactly the same as kun menet.
Could I include the word sinun before mennessäsi to explicitly say 'when you go'?
You can, but for personal pronouns it is much more natural to drop the pronoun and just rely on the possessive suffix (-si). Note that the sinun later in this sentence is actually the subject of kannattaa, not part of the temporal construction!
Why is the object matkakortti in its basic form rather than having an -n ending?
The main verb of the sentence is kannattaa, which acts like a necessity verb (similar to täytyy or pitää). When you have an impersonal necessity structure like sinun kannattaa (it is worthwhile for you / you should), the object takes the basic nominative form instead of the -n accusative ending.
Why do we use the illative case metroon (into the subway) instead of metrossa (in the subway)?