The most effective way to learn Portuguese grammar is by memorising sentences’ structure, and how to conjugate verbs and repeat them until it comes out naturally. Grammar is probably the area of language learning that is most similar to Maths. Essentially, you will need to memorise how the language structures itself and repeat as many exercises as possible until you don’t actively think about it and it comes naturally when you speak and write. It might be useful to compare Portuguese Grammar to your language’s Grammar. By identifying the differences between the two you will be able to better understand how Portuguese works.