Not "I like the coffee. (which coffee? the one we both know)" — say "I like coffee. (all coffee, in general)". a = 很多当中的一个 · the = 我们俩都知道是哪一个 · 什么都不加 = 泛指。
Ira wants a sandwich. The sandwich is big.
Liam likes coffee.
Sandwiches are cheap here.
a sandwich — one of many, a new one
the sandwich — you both know which one
coffee — coffee in general
泛指一类东西
说的是一整类东西、泛泛而谈（所有的，哪儿的都算），就用复数，前面什么都不加。
Sandwiches are cheap here.
Liam loves cookies.
Apples are good for you.
Not "I like the apples. (which apples? the ones we both know)" — say "I like apples. (all apples, in general)". 一整类、泛泛而谈 → 通常什么都不加。
试一试: Ira 说的是苹果这一类东西，所有的苹果，到处的苹果。她说："___ apples are good for you." 哪个词合适？ (nothing / the) 答案: nothing — 泛指一类东西、所有的苹果、到处的苹果，前面什么都不加：Apples are good for you。The apples 会变成你们俩都知道的那几个具体的苹果。
回到具体的那一个
泛指一类东西，什么都不加。但说的是眼前这一个，就要把 the 加回来。
Liam likes coffee. → all coffee, in general
The coffee here is very good. → this cafe's coffee
Ira loves apples. → The apples on this table are sweet.
Not "This cafe makes good coffee. "Coffee here is very good."" — say "This cafe makes good coffee. "The coffee here is very good."". 泛指 → 什么都不加。具体的那一个 → the。
试一试: Ira 说的不是所有的茶，她说的是这家咖啡馆的茶。她说："___ tea here is very good." 哪个词合适？ (The / nothing) 答案: The — 她说的是这家咖啡馆的茶，不是全世界的茶，所以 the 又回来了：The tea here is very good。
Not "I like a coffee. (you can't count coffee, so no a)" — say "I like coffee. (words you can't count take nothing)". 数不了的词，比如 water、coffee、rice → 不加 a，直接用这个词。
试一试: Liam 说的是他每天喝的水。他说："I drink ___." 哪个合适？ (a water / water) 答案: water — 水数不了，所以我们从不说"a water"。数不了的词，比如 water、coffee、rice，前面什么都不加：就说 water。