Tämä viikko on ollut todellinen haaste, koska minulla on monta koetta.

This week has been a real challenge because I have many exams.

Breakdown of Tämä viikko on ollut todellinen haaste, koska minulla on monta koetta.

• minä: I
• olla: to be
• tämä: this
• koska: because

Questions & Answers about Tämä viikko on ollut todellinen haaste, koska minulla on monta koetta.

Why is it "Tämä viikko on ollut" and not "Tämä viikko oli" or just "Tämä viikko on"?

All three are grammatically possible, but they mean slightly different things:

• Tämä viikko on ollut todellinen haaste
  = This week has been a real challenge.
  This is the present perfect. It focuses on the whole week up until now, and often implies the situation is still ongoing (the week is not over yet, or its effects are still felt).

• Tämä viikko oli todellinen haaste
  = This week was a real challenge.
  Simple past. The week is clearly over; you’re looking back at it as something completed.

• Tämä viikko on todellinen haaste
  = This week is a real challenge.
  Simple present. You describe the current state, but less strongly as a “from the beginning up to now” experience. It sounds a bit more static, like a general comment about this week.

In everyday speech, "on ollut" is very natural when the week is still in progress and you want to emphasize the whole experience so far.

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Why is it "Tämä viikko" and not "Tällä viikolla"?

Both are correct, but they focus on slightly different things:

• Tämä viikko on ollut todellinen haaste
  Literally: This week has been a real challenge.
  The week itself is the subject – you are talking about the week as a period and characterizing it.

• Tällä viikolla on ollut todellinen haasteLiterally: Here (, adessive case) is a , not the subject. The sentence really says: