When learning English grammar, focus on the basics, such as the parts of speech and tenses, etc..
Get a general understanding - do not worry about memorising 100% of the important data - with time, you can try to remember more if necessary.
You do not have to learn everything in English, so that you can produce it accurately - a lot of English is about using what you know and understanding what you receive.
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When learning English grammar, one question that some may wonder about is: "Which grammar should I learn?". When I started teaching English by Skype in January 2007 (over 8 years ago), I had previously been teaching for about 1 year offline, as well as having passed an English Literature Major and additional Creative Writing Minor from a high-level UK university course, so thought that I knew a reasonable amount - after all, English had always been a strong subject at school and surely there was not that much to know, having already taken 4 teaching courses: most of them quite short.
After starting teaching, I quickly found that at least one of my students was very high level compared to my knowledge and realised that I would have to start learning grammar much more intensively. Subsequent to that, I reviewed topics before teaching students and also started a newsletter which focussed on grammar topics. Having now taught numerous grammar classes by Skype, and even one or more students from zero grammar to covering numerous grammar topics, I now feel that I know a lot more - however, having looked through some British Council etc. grammar sites, I still realise that what I learned is maybe nothing compared to the numerous grammar rules that could be learned - after all, English has a million words and most native speakers know a small percentage of them and a percentage of these words may have specific extra rules, etc..
So, the question then is "What should I learn in grammar, if there is too much learn it all?" - my opinion is that very few if any English grammar teachers in the world know everything that there is to learn. So what to learn? Assuredly, UK native speakers like myself will likely understand a lot of grammar, even if not being able to know all the rules, but what about English learners?
From my years of English teaching experience, I try to start students with learning the 8 parts of speech (some grammarians might have a different total count) and also the 12 basic tenses of English. Clearly, there will be a much higher requirement to learn the "Present Simple", than the "Future Perfect Continuous". Moreover, for each of the tenses there is a lot more to remember for specific details. The tip that I give students is firstly to not worry about memorising all the data - having taught grammar for years, I certainly cannot remember in detail all that I have learned and now teach and it is still helpful often to refer to grammar data, although the more I teach, the more I have remembered.
Second, just focus primarily on the usage/structure etc., whilst not trying to learn all examples, etc.. There is a huge amount of potential data to learn and there "are 24 hours in a day". The method that I like to teach students in classes, is to "become familiar" with the basic concepts of grammar - the "fine detail" can come later as is required through practice and correction. Thirdly, "try to go round a mountain, not over it" - whilst it is great to be able to produce any grammar required, that is most often only going to be able to be done by higher-level native speakers - try to focus on being able to use a sufficient amount for your needs and being sufficiently familiar with the rest to generally understand what is going on and being able to know where to refer to, should you need to check on something you have read or may want to produce in future.
Finally, from my (currently in January 2015) approximately 9 years of teaching English, here is a list (a suggestion, rather than anything fixed), that I may work through with my students, looking at the main areas being: "parts of speech and the 12 basic tenses":
1. Parts of speech.
2. 12 Basic tenses.
4. Modal verbs.
5. Phrasal verbs.
6. Passive/Active concepts.
8. Uncountable/countable nouns.
9. Ways of talking about the future.
10. Independent/subordinate clauses.
Clearly some may have specific issues, such as the use of punctuation etc. and this just an example - just like going to a doctor, they treat you for the issues you have, a skilled teacher can focus in on the weaker essential areas, whilst ignoring those that are less important - just as it is not possible to have a medicine that is likely to cure all illnesses, so too should a student and teacher focus on the weaker areas, rather than trying to cover things already understood clearly.