Learning grammar is straightforward: Pick a grammar guide and read it. Tae Kim is often recommended for beginners (note that the whole guide is basic grammar, even the “advanced” section), but other options are listed on the resources page. You shouldn't expect to memorise everything you read the first time around in whatever guide you choose, but you should be aiming to understand it. The purpose of a grammar guide is not to grant you "mastery" over the language (which only comes through lots of practice and exposure), but simply to introduce you to the fundamentals of the language and give you the foundation that you need to start reading native material.
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