Suggestions for learning polish?
I dont really think duolingo is good for polish like it is for spanish. Im constantly missing all the plural nom female and singular male instrumentive etc. I think its probably best to read some polish grammar books first before i take duolingo polish seriously. Like should i memorise every table and every form of tamci tego te etc before i learn vocab or translation?
YouTube and Memrise for Polish as well as Spanish. Both will help, along with Duolingo of course. Polish is gonna be a challenge the first few months, as with any Slavic language, but you'll get it.
Certainly, it doesn't hurt to have a good Polish textbook and use it simultaneously with the course on Duolingo. But you can also find quite good resources about Polish on the Internet. Check for example these: http://seelrc.org:8080/grammar/pdf/compgrammar_polish.pdf, https://www.duolingo.com/comment/13262257, http://mowicpopolsku.com/.
And no, you shouldn't try to memorize all the possible forms of a new word. Even if you managed to learn them, the process in which you would try to analyse which form you should use and then trying to pick out this proper one is not efficient. All the words follow some more general pattern and the more time you devote to learning Polish, the more obvious and natural it should become. It's better to memorize the whole sentences or even the whole texts. In this way you not only learn new words but you also learn how to use them correctly in different contexts.
I've been lazily learning Polish (that is, listening to lots of music, radio, occasional reading and grammar study) for probably 7-8 years before starting Duo Polish, and I find it really helpful for grammar at my mid-intermediate level. That said, I can see how approaching the course as a total beginner might be overwhelming. Swan's "Polish Grammar in a Nutshell" is an excellent reference (http://www.skwierzyna.net/polishgrammar.pdf). However, I wouldn't kill yourself memorizing grammar at the expense of learning vocabulary, which is arguably more important. I would advise you to find music you like in Polish, listen and read the lyrics, and look for opportunities to learn vocab+grammar together. For me, hip hop works really well because it's easy to memorize rhymes, and you can also commit to memory prototypical examples of grammatical structures. For example, "Ta dolina" means " this valley," and there is a line that ends, "...będziesz sam w tej dolinie" ("you'll be alone in this valley"). If I remember "ta dolina" becomes "tej dolinie" in instrumental case, I know this "a" changes to "ie" applies to most feminine nouns, so "ta kawa," "Co jest w tej kawie?"; "ta strona", "Znalazłem to w tej stronie," etc. There are exceptions (kobieta - kobiecie ('-ta' - '-cie'); also others), but I'll be right most of the time.
This is one of those situations when asking a native speaker will help. Without actually using these forms, it will be hard for you to know when to use them. When the immersion function becomes possible through duolingo, that should help with grammar. Also, formal Polish is much more specific than informal. You can get by a lot more by using the informal words and rules, but that will only be learned through conversation with other Polish speakers.
I have asked many Polish speakers questions about their language and I have learned that Polish speakers don't know the rules for their own language well enough to explain them, they have just learned through listening and speaking at school. Even one person I speak to who excelled at Polish language in school struggles with some questions I ask. After two years of learning using apps I now have a vocabulary of around 2000 words but struggle to decline a simple sentence. There appear to be so many exceptions to Polish. My goal is to to be able to have a conversation entirely in Polish within 10 years...only 8 years to go, so not sure I will manage that :)