If you want to learn the right words and spellings or want to remember them better, you need a dictionary. But if you are looking for a more comprehensive one than the standard ones out there, you need Concise Oxford Thesaurus for Android. 

This is the best app for the best English dictionaries you will ever need. It contains over 300,000 definitions, synonyms, spelling variants, and phrases. It’s great for grammar and writing. The app has a search engine, so you can search for a word or for the word in its synonyms. If you know what you want to look up, just tap the word and select the synonyms or related words to get all of them. You can also search for the spelling of a word.