請教一個英文法問題，如右 "Bob tells his wife Susan that he lost his job. Susan suggests that he start his own business." ； 句中的start應是第三人稱動詞，為何字後面不用加s呢?

此句原來應該長這樣:
Susan suggests that he SHOULD start his own business.

像Suggest這種動詞後面子句需要用
that+S+should+原型V
但是should可以省略，因此就會只剩下原型動詞了

以下是常見的相同用法的動詞
建議:advise,suggest,recommend
堅持:insist
要求:demand,urge
命令:command, order
再舉個例子:
Her mother demands that she (should) play the piano twice a week.

https://www.grammaring.com/present-subjunctive

Present subjunctive
The present subjunctive is identical to the bare infinitive form of the verb in all persons, including the third person singular (no final -s). It is usually used in formal or literary styles:
Present subjunctive在所有人稱(包含第三人稱單數)，均使用不帶to之動詞原型，這種句型通常用於正式與文學寫作
…
After verbs such as INSIST, SUGGEST, RECOMMEND etc.
通常用於類似INSIST, SUGGEST, RECOMMEND等動詞後
Mike insisted that I try his new muffin recipe.
I suggest that your cousin apply(第三人稱單數，動詞也不+s) at once.
Carl was injured last week, and the doctor recommended that he not play(第三人稱單數，動詞也不+s) in the next match.
Again, the present subjunctive can be replaced with should + infinitive in less formal styles:
這種不帶to不定詞，可用 should + Verb 取代，為比較不那麼正式寫法
I suggest that your cousin should apply at once.

Amendment: Bob tells his wife Susan that he lost his job and then she suggests that he ought to start his own business. Yip

that he "would" start his own business---conditional clause; subjunctive mood.

.

should [助動詞]：釋義-1. ((that 節において，仮定法または仮定法相当語句を導く))
(1)((驚き・意外・遺憾・善悪［当否］の主観的判断などを表す主節の後に))
例1. It is surprising that he should do a thing like that.
例2. It was a great pity that James Dean should have died so young.
例3. It's only right that he should claim payment for the damage.

◆should に代えて直説法や((米)) では原形仮定法現在も用いられる
例. Do you think it's normal that the child is so tired and inactive?

(2)((英)) ((提案・勧告・要求・主張・意図・決定・決心・命令・規定などを表す主節のあとに))
例1. She recommended that her husband (should) stop drinking for a while.
例2. I insist that she (should) tidy the room at once.

◆((米)) では should を省き，原形仮定法現在を用いる. 現在では((英)) でもこの傾向が見られる. くだけた言い方では直説法も用いられる.that と should が共に落ちた型もある
例. I strongly recommend you not carry too much cash.

Susan suggests that he start his own business.
這一句有簡寫
原文應該是Susan suggests that he should start his own business.
should-->應該