It depends from where you will start or i can say what is your knowledge in English grammar is.

Or if you are preparing for nay competitive exam then i will suggest you to be certain about your exam.

1.LearnEnglish Grammar (UK ed.)

2.Johnny Grammar Word Challenges

3.English Grammar in Use: Sample

4.Grammaropolis - Complete Edition

Above app can help you

Hello Aspirant!

*Improve English: Vocabulary, Grammar, Word Games

This is one of the very best app for improving your grammar and vocab.

* Merriam Webster

This app serves as a very good dictionary as well as Thesaurus

You can also use an online platform named Grammarly.. That will help you in correcting your grammatical errors while writing something!