請問幾時用have,has; 幾時用do,does?

have/has 解「擁有」，第一二身（I, you）或多於一人（we, you, they）用 have、第三身（he/she/it）用 has，呢個唔駛詳細講啦。但如果你講o個句唔係 present simple tense，咁就要諗諗。
如果你想講 present perfect tense，則由於句式係 have/has + past participle，咁就要睇住句子主詞係第幾身，o係 have/has 中選用適當o既一個。
而 present perfect 用於講一ｄ剛發生、或者發生左而影響維持至今o既事。如你o既例句：
「我沒有買這件玩具」，影響維持至今（今依家都冇呢件玩具），所以可用 present perfect 講，即係：
I have not bought this toy.
但o係美式英文之中，其實好多時會以 simple past tense 取代 present perfect，於是就可以講為：
I did not buy this toy.
又如果其實件事已經發生左好耐，同現今冇乜關係，咁你亦可以用 simple past tense 講。
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至於 do / does，用法都係睇主詞第幾身（同上）。至於幾時用，其實一般只係o係問問題時用。有ｄ動詞可以作為問題開頭，如：can/could/may/might/will/would/shall/should/must/ought，咁問題就可以咁問：
Can I get the points?
但有ｄ動詞不能咁用，於是就要加個可以開頭，但加左又唔會改變句意o既動詞入去。呢個就係 do。如：
You like my answer. => You do like my answer.
呢個 do 其實可以加入任何句子o既動詞前中，作為加強語氣用，但留意加左後佢就成為主要動詞，第三身同時態變化就去哂佢度，ｎｏｔ都去埋佢度，如：
You DID like my answer.
He DOES NOT like my answer.
有左佢就好易搞啦，將佢調上開頭，就成為問題：
Do you like my answer?
Did you like my answer?
Does he not like my answer?
當然，ｄｏ可以作「做／幹」解啦，咁就同以上解釋無關啦，但一樣可以加 do ，如：
I did do it. => Did I do it?

ok

For do,does: I do not buy this toy. Buy ---verb,so use do or does.
For have or has: have or has單獨使用 or 後面follow past participle For example: I have this toy but my friend has not this toy.我有這玩具但我的朋友沒有 Or I have bought this toy for three days. 我已買了這件玩具
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