Naturally, it’s important that you understand what is being put in front of you. If your Spanish isn’t up to scratch, make sure you find somebody to read over the contract before you sign. It’s also important that you trust the landlord and agent.
Rental contracts in the Dominican Republic usually last for one year - though rolling monthly contracts can be negotiated (a good idea if you are new to the island). You might be required to allow your landlord access to your accommodation on a monthly basis to check everything is in order - this should appear as a clause in the contract. The landlord is also required to fix anything in the apartment which has not been broken by the tenants.
It’s not unusual for landlords to demand 2+1 or 3+1 of the equivalent of the monthly cost of rent. What this means is that two to three months rent will be paid as a deposit plus an additional one month's rent as a fee towards the landlord’s lawyer. The proportion that makes up the deposit will usually be placed into a bank account until the end of the lease, when you will receive it back.