说到一样东西之前，我们会说 a 或 the。a = 很多当中的一个，新提到的 · the = 我们俩都知道是哪一个。
a 还是 the
Lani has a dog. The dog is small.
There is a cafe here. The cafe is very busy.
Look at the sun!
Not "Lani has the dog. (which dog? we don't know yet)" — say "Lani has a dog. (one of many, a new one)". a = 很多当中的一个，新提到的 · the = 我们俩都知道是哪一个。
Lani has a dog. The dog is small.
There is a cafe here. The cafe is very busy.
Look at the sun!
a dog — one of many, any dog, a new one
the dog — you both know which one
第一次说到它
第一次说到一样新东西时，用 a 通常最自然。之后再提到它，就用 the 指回去。
Ethan has a blue bag.
The bag is very big.
There is a new pool here. The pool is warm.
Not "It is the first time. "There is the pool here." (but we don't know which pool yet)" — say "It is the first time. "There is a pool here." (a new one, one of many)". 新东西、第一次 → a。已经知道是哪个 → the。
试一试: Ethan 打开书包。这是他第一次提到自己的书。他说："I have ___ book." 这里用哪个词？ (a / the) 答案: a — 第一次、一样新东西 → a book。"The book" 会让人以为我们已经知道是哪一本了。
我们都知道的那一个
你和对方都知道是哪一个，就用 the——就是眼前这一个，这个房间里、这个地方的那一个。
The door is open.
The pool is next to the gym.
Lani is at the cafe.
Not "There is one door here. "A door is open."" — say "There is one door here. "The door is open."". 我们都知道是哪个 → the。
试一试: Ethan 和 Lani 在同一个房间里。房间只有一扇窗。Ethan 说："___ window is open." 哪个词合适？ (The / A) 答案: The — 这个房间只有一扇窗，所以你们都知道是哪一扇。用 the。"A window" 是"很多扇当中的一扇"，但这里只有一扇。
只有一个
有些东西世上只有一个。只有一个 sun、一个 moon、一个 sky。它们永远用 the。
The sun is very bright.
The moon is white tonight.
The sky is blue.
Not "A sun is very bright." — say "The sun is very bright.". 世上只有一个 → the。
试一试: Lani 抬头看。月亮只有一个。她说："___ moon is very big tonight." 哪个词合适？ (The / A) 答案: The — 月亮只有一个，所以我们总是说 the moon。"A moon" 会变成"很多个月亮当中的一个"。