Problem about English Tense

The first five pairs of sentences are grammatically correct.
For the last pair of sentences, I have three suggestions.
(1) I do not really know what you mean by "exercising skills". I think "exercising myself" is a better choice
(2) "When" is fine but "While" is better as it is followed by the past continuous tense
(3) It is better if the word when/while is placed in the middle of a sentence instead of at the beginning though placing it at the beginning is not wrong

句子唔單係tense正確便是正確和合理的句式。雖然所有句子用past tense都沒有用錯，但句子的組織和寫法就問題多多。只能建議如下給少少講解，未能全面說明。 Sam fell from a ladder yesterday. He felt pain. 尚算可以，留意第二句表達的相對時間：是當時feel pain還是到「今天」(昨天之後)才/仍feel pain。如可表達因果時序關係才算是好句，否則只是兩個無關的句。 The bomb in ABC City exploded at 3:30pm yesterday. Thirty-nine persons died.
錯了幾個地方都跟tense無關的。改正作解釋：A bomb exploded at 3:30pm yesterday in ABC City. Thirty-nine people died.你寫the bomb表示你已知道是那一個bomb，文理上不合理，除非這句之前已經交代了製作這個the bomb，否則像新聞報導的句子是會用a bomb exploded / a car crashed / a person injured而不會用the。句式的文法bomb(subject) exploded (verb)是要連續的，像你的寫法”subject” in “object” “verb”其實是把object變為subject，意思是「有數個城市的指定炸彈爆炸」才會這樣結構：第一個是ABC城的爆炸….第二個是xyz城的爆炸….第三…。persons這字的用法是指一個個體的人，而數人數量時要看你是表達人的量值還是人的角色。這裏雖然是數人數，但「人」這個字的用意不是說「人頭/人次/每個人」而是一類人(死了那批)，所以英語人會用people來歸類，而不是數人數那種寫法：the meeting has 12 persons attended. The flyover in ABC City collapased[串錯] at 9:00pm last night. Forty-five persons fell from the flyover.
同上！ jewellery[串錯 I played tennis yesterday. When I was playing tennis, I was exercising skills.後句句式全錯。Skills本身沒有指定意思的…你要指明是那種技能。假如你想表達練習球技 – 是用practice這動詞和technique這個名詞I practiced various tennis techniques while playing.

Sam fell from a ladder yesterday. He felt pain.

Basically, all sentences are grammatically correct except the last one, which sounds funny.

1) Mary was unhappy because of her mother's death yesterday. I comforted her.- OK

I comforted Mary because she was unhappy at the death of her mother yesterday.


2) The bomb in ABC City exploded at 3:30pm yesterday. Thirty-nine persons died.- OK

Thirty nine persions died duing the bomb in ABC city exploded at 3:30pm yesterday.


3) The flyover in ABC City collapased at 9:00pm last night. Forty-five persons fell from the flyover.- OK

Forty five persions fell from the flyover in ABC city when it collapsed at 9pm last night.


4) Three robbers robbed a jewellery shop in MongKok at 5:30pm yesterday. Three staff of the jewellery shop died. = ok

Three staff of the jewellery shop died when three robbers robbed the ship in Mongkok at 5:30pm yesterday.


5) I played tennis yesterday. = ok
When I was playing tennis, I was exercising skills.???

I played tennis to better my skills yesterday.