Knowing part of a password will surely make it easier, from a mathematical/theoretical perspective, even if you don't know the length. Basically, you can consider that whole segment of the password which is known to be a single character which the hacker knows.
E.g. consider a 10 digit password that accepts only 0-9 as input (for argument's sake). This password will have 10^10 possible answers. Whereas, if we know that one is a 1, there are 10*(9^10) (which is ten times nine raised to the ten), as far as I understand. Please correct me if I'm wrong.
Edit: I was wrong. There are actually s^l possible passwords, where s=the number of symbols and l=the length of the password. This is for a fixed length password, but for the sake of argument, I'll use it anyway (if it's variable length, then it's s^1+s^2+...+s^l where l is the max length). The reason that it's s^l is because for each position there are s different symbols that could be there. s*s*s...*s (that is, s times s, times s, times s ...., times s) l times are the total number of possible combinations for each length l.
Back to the example, if one of the characters are known, but you don't know the position, it's really just 10^9 different guesses, because there are effectively only 9 positions. Because you always know what's in the remaining one!
tl;dr this definitely makes it easier from a guessing perspective: if you know one chunk of the password, you remove a big chunk of the guesswork.