Two factors determine your rent amount for the first and the last month of your stay.
The contract type set by the landlord.
Your move-in and move-out dates.
Contract types
Daily contract — you pay rent for the number of days you book.
Fortnightly — in a month, if you stay for 14 days or less, you pay half that month's rent. If you stay more than 14 days, you pay the entire month's rent. Any excess rent will be reduced from the following month (if there is one).
Monthly — every month, you pay the entire month's rent.
First month's rent
Regardless of the contract type (daily, fortnightly, or monthly) or move-in date, you will always be charged the entire first month's rent.
Because you will pay the entire first month's rent to book a place, any excess rent you pay in the first month is reduced from the second month's rent.
Examples
You're looking to book a place from 20 April - 10 August. The place you found has a rent of €1000.
For the daily contract type, you only pay for the 11 days you stay in April. Because you paid the entire month's rent to book the place, the excess amount paid for April will be reduced from the rent for May. In June and July, you'll pay the monthly rent as usual. In August, you'll pay rent for 10 days.
For the fortnightly contract type, you pay half the rent for the second half of April, the entire month's rent for June and July, and half the rent for the first half of August.
For the monthly contract type, you pay the total monthly rent for every month you stay.
Breakdown of the rent on a daily contract
Breakdown of the rent on a fortnightly contract
Breakdown of the rent on a monthly contract
You can see how much you'll pay during your stay at every stage of the booking process.