Duolingo

Depends. I've been trying to learn Spanish and I can pick up some phrases and reading is a little easier. Not great though. I've heard about problems with Duolingo not picking the right words for some phrases. It would still convey your idea but sound odd.

The problem is recall and consistency. You need to remember, actually try to remember. Don't get in the bad habit of guessing everything and not really understanding. The skinner box type app like these have a paid version that lets you make all the mistakes and keep going. This can help form bad habits if you aren't trying to recall the information. You will just jump to the wrong answer because that's the one you are used to picking. Write down notes after/during the classes. You get more out of it if you put more in it. It is a tool and not THE tool. Use other apps, website, tv shows, radio stations, or anything else. You have to practice.