First of all: Both the email and the password are case sensitive - make sure they're entered with the capitalisation used when signing up.
If you signed up using Facebook, always use your Facebook as login. If you forgot your password, you have to ask Facebook to reset it.
If you signed up using Google, always use Google as login. If you forgot your password, you have to ask Google to reset it.
If you signed up using your email, always use email when you log in. If you forgot your password, we can reset it for you. Just click 'forgot password'.
You can always add a password login to your account by requesting a password as well.
So these tips work for adding a password to an existing account, and for resetting a forgotten password.
Tap 'Forgot password for email?'
Enter your email address. If you don't remember which email address you used for sign-up, check your email archive for emails from firstname.lastname@example.org
If we have your email address on file we will send a reset link. The mail is sent from email@example.com with subject line Reset your password. Make sure to check the spam folder if you can't find it. The link is only valid for 24 hours.
Reopen the app and sign in with your email and new password.