Well, for the example you stated, it's better to say أشرب الماء. There are some rules that can lead you when to use "ال", but in other situations it comes with practice. - One rule is that when a noun comes at the beginning of the sentence e.g. الرجل يشرب or الأكل لذيذ (food is delicious). - Another rule is adjectives that come after a noun that has "ال" in front of it. Adjectives in Arabic always come after the noun, and it follow the noun in: 1- gender 2- number 3- the presence and absence of "ال". So you can say الرجل الطويل سعيد (The long man is happy) or أنا استمعت إلى الأغنية الأولى (I heard the first song) - Some verbs prefer to have an object that has ال in front of it (in case that the object is a noun), e.g. أنا أشرب الماء and هو أكل الطعام , but if you are planning to add an adjective to the object then you can use or drop the ال e.g. أنا أشرب ماءاً نقياً or أنا أشرب الماء النقي ( I drink pure water) هو أكل طعاماً لذيذاًً or هو أكل الطعام اللذيذ There may be a slight difference in the meaning though, when using ال here, because without ال then you mean "in general" and with ال you may mean in a particular situation. - Sometimes we add ال to a noun to generally speak about all of its kind, as to make a general statement. e.g. إنسان ضعيف (a weak person) هذا إنسان صعيف (This is a weak man) but الإنسان ضعيف (Men (humans) are weak) to make it more obvious we will use a verb: كان إنسان ضعيف (He was a weak man) كان الإنسان ضعيف (Humans were weak) Another example of using ال in some cases when talking in general: سأستيقظ باكراً غداً (Tomorrow I will wake up early) but سأصبح الغد طبيباً (Tomorrow (one day/in the future) I will be a doctor)

if it is known to you and known to the listener use ال ذهبت إلى المدرسة if it is unknown to you or you want to keep it unknown for the listener or it is not important to be identified don't use ال ذهبت إلى مدرسة that is in general for water you have the choice to use it or not, because water is known already أريد أن أشرب ماءاً= أريد أن أشرب الماء but if you are talking about a specific water( let's say that you are keeping cooled in the fridge and you have been talking about before), then use ال

the second one is correct. Or you could say اشرب ماء الصنبور or اشرب ماء الواد. these are just example. to get rid of ال, you need to add something that brings "precision".

Both of them is correct with a slight difference in the meaning. 1-(أشرب ماءاً)(أشاهد فيلماً)I drink water not tea,I watch a film not something else. 2-(أشرب الماء)(أشاهد الفيلم)you are referring to a specific film or water and your partner knows exactly which film,water.