もしかしたら

もしかしたら means "maybe", "possibly": the speaker raises something as a real chance without being sure of it. It goes at the start of the sentence, and the sentence nearly always closes with かもしれない or かもしれません.

もしかしたら or たぶん

たぶん means "probably": the speaker thinks it is more likely than not, and the sentence ends in だろう or でしょう. もしかしたら is a smaller chance, and it takes かもしれない.

もしかしたら opens the guess, but the end of the sentence still has to be uncertain. A flat statement after it does not work.

• × もしかしたら彼は来ない → もしかしたら彼は来ないかもしれない

In conversation the end can also be a question to yourself, んじゃないかな.

Usage notes

These go beyond もしかしたら itself.

もしかして in questions

もしかして puts a guess to the listener: "by any chance". It belongs to speech, and the sentence is usually a question.

もしかすると

もしかすると means the same as もしかしたら and sounds a little more formal. You meet it more in writing.