Which language are you trying to learn? There are different answers depending on that.

As someone learning Hindi, I've found that Duolingo is wholly insufficient in grammar and vocabulary (the entire course is far too short) and did not concentrate on listening comprehension. I've started using a combination of the following:

• Clozemaster for vocabulary in context of (sometimes pretty wild) sentences. (I've got a lifetime subscription to Clozemaster, it goes on sale during holidays.) Clozemaster has grouped "common words" and a combination of reading/listening skill and multiple choice / vocab word transcription / entire sentence transcription. It feels very overwhelming at first as you're just thrown in but keep at it - start with reading and multiple choice and once you know the words and sentences in your grouped section start typing them out via listening.
• A combination of textbooks and websites to explain certain grammatical concepts.
• A listening-based podcast, example Innovative Language, for listening comprehension. (This also goes on sale regularly.)