原句含有情态动词，引用的时候情态动词有时会产生变化，具体规则如下：
B类could, would, should, might, needn't, ought to, used to, could have, should have等引述时不发生改变
例：原句：'I could meet you at the airport.' 引述句：He said that he could meet us at the airport.
原句：'We might drop in if we have time.' 引述句：They said they might drop in if they have time.
原句：'You should have contacted me earlier.' 引述句：She said I should have contacted her earlier.
C类will, can, may，引用时通常改为would, could, might，
例：原句：'She may have already left.' 引述句：He thought she might have already left.
原句'Careful! You'll fall through the ice!' 引述句：I warned him he would fall through the ice.
但如果引用主句为一般现在时，且事件或情况依然存在或会延续下去，则情态动词不发生改变，如果引用主句为过去时，且事件或情况依然存在或会延续下去，则情态动词可不变或变为would, could, might。
例：原句'I'll be in Paris at Christmas.' 引述句：She tells me she'll be in Paris at Christmas.
原句'The problem can be solved.' 引述句：They said the problem can/could be solved.
D类shall，引用时变为would谈论将来，should表达帮助、要求或建议等
例：原句'I shall (I'll) call you on Monday.' 引述句 She told me she would call me on Monday.
原句'Where shall I put this box?' 引述句：He asked where he should put the box.
E类must译为必须时，引述句为must或had to，must译为总结时，引述句为must，mustn't不发生改变
例：'You must be home by 9 o'clock.' 引述句：She said I must/had to be home by 9 o'clock. （口语中had to更自然）
'I didn't hear the phone. I must be going deaf.' 引述句：Alex thought he must be going deaf.
'You mustn't tell my brother.' 引述句：He warned me that I mustn't tell his brother.
注意有时原句中没有情态动词，引用句里可以加上
例：'You're not allowed to smoke here.' 引用句：She told me that I mustn't smoke there.
'My advice is to look for a new job now.' 引用句：She said that I should look for a new job now.