請問would在這句裡是什麼意思?

Would
- used for talking about things that often happened in the past.
(SYN)used to:
When my parents were away, my grandmother would take care of me.
H'd always be the the first to offer to help.
(quoted from: Oxford Advanced Learner's Dictionary)

"When he returned to the martial art Daniel was a lot weaker than he was before his illness and his class mates and teacher would hold his hands to help support him."

Here, the use of the word, "would" , means his teacher and classmates used to hold his hands, or they often held his hands to show support. So you can imagine the holding-hands scenario happened more than just one time.
If you say, "they held his hands", probably you are describing something happened on that very day when he came back to school.


2013-12-28 17:55:39 補充：
he was a lot weaker 是狀態, 不是動作, his classmates's holding his hands 是動作. 此處的 and 不是表達時間差. 比如這句, I am too old and I will die soon. 我已經太老了, 而且我不久將會死去. 如果翻譯成 我太老了, 接下來我將死去. 就有點瑕疵. 表達時間差的 and 會是兩個動作: He went home and took a shower.

would在此用來表示一個過去的未來狀況，因為其時間點在另一個過去事件之後，但也是屬於過去的事件。兩者有時差。

2013-12-28 16:26:30 補充：
he was a lot weaker and they would his hands 表示有時差：一先一後。

可參考
http://english.nhsh.tp.edu.tw/p21-14.htm (would部分)
http://englishhome.org/archives/3911 (全)

would這裡表過去習慣
通常可以翻成：通常都會.....

would hold=auxiliary verb+verb;the basic use of an auxiliary verb is to help a principal verb to form into its tense, mood and voice in literature of elegant writing.
to help support=help=support;
help=a verb transitive,
support=another verb to form a compound verb
to help support=an infinitive clause.
The infinitive clause with the verb help form into its tense, mood and voice to form an elegant writing;not as simple as held his hands.

would hold his hands to help support him.

(會) 握住 他的 手 以 幫助 支持(鼓勵) 他
這裡翻成(會)

...his class mates and teacher held his hands...
直接寫是也可以，但是太直接，不夠婉轉，缺乏文字優美度