Grammar for $\{ a^i b^j: j < 2i \text{ and } j \ne i \}$

For the following language, write grammar independent of the text.

$$\{a^i b^j: j < 2i \text{ and } j \ne i \}$$

I want a hint to start solving this problem. Where should I start?

If $$j \neq i$$ then either $$j < i$$ or $$j > i$$. Hence you can write your language as $$\{ a^i b^j : j < i \} \cup \{ a^i b^j : i < j < 2i \}.$$ It suffices to find a grammar for each of these two languages. Such questions have been asked and answered on this site.