help me ,please.

1.no bread eaten by man is so sweet as that earned by his own labor 为什么用that?one 或 what 为什么不行?
首先,bread是不可数名词,用one肯定是不行的.其次,这是一个定语从句,如果还原为正常语句应该是:No bread eaten by man is so sweet as the bread earned by his own labor.第二个bread是先行词,它后面是修饰它的定语,改为定语从句的时候用that代替了the bread,是合理的.
2.you can not imagine the hard time he had holding the horse in
为什么用holding?怎么翻译?
hold在这里是“拉,牵”的意思,就是牵马.he had是time的定语,time是先行词,hard time就是“艰难的时间”,you can not imagine the hard time he had意思就是：你无法想象他那段艰难的时间,是什么时间呢?holding the horse in,牵马进来的那段时间,这也是修饰time的定语.整句就是：你无法想象他是如何艰难地把马拉进来的.这里其实是省略了when he was,如果加上你可能就看得明白了：you can not imagine the hard time he had when he was holding the horse in.
3.I have kept the portrait where I can see it every day,as it always remind me of my university days in London 为什么用where?in which为什么不行?
在定语从句中,in which确实常常可以代替where,可问题是这里并不是定语从句,而是状语从句,否则的话,你告诉我先行词是什么呢?换回正常句型的话,应为：I have kept the portrait there.我把画像放在那里,放在哪里呢,来一个状语从句进行说明.如果原句是：I have kept the portrait in the closet where I can see it every day,这时where可以换成in which.
另外有个友情提醒,并不是说在状语从句里in which就可以代替where,有时候是用on which,或其他介词,比如：I have kept the portrait on the wall where I can see it every day,这里where就只能用on which 代替.
4.it is quite warm today for january.为什么用for?怎么翻译?
for 是对于的意思在这里,整句就是说：今天的天气对于一月份来说算相当暖和的.