Partly means to some extent but not completely. When something is partly true, it's true in some ways but not others. When you're partly finished with your homework, you've done some of it but still have more to go.

If your mom asks why you're late and you say the bus was partly to blame, you're admitting the bus delay was one reason, but maybe you also spent too long talking to friends. When a basement is partly flooded, water covers some of the floor but not all of it.

The word helps you be precise about situations that aren't all one way or all another. A movie might be partly funny and partly scary. A project might succeed partly because of hard work and partly because of good luck. Your room might be partly clean if you picked up your clothes but left books scattered around.

Partly is similar to partially, and you can usually use them interchangeably. Both words acknowledge that life rarely comes in simple absolutes. Things are incomplete, mixed, or somewhere in between, and partly gives you a way to describe that reality accurately.