1. however表达“然而”的意思时，是副词，不是连词（表达“不管用什么方法”的意思时是连词），不能直接跟在逗号后面，而要跟在句号或分号后面；另外，however后面一般要加上逗号。

可以写成：

The clinical response to treatment was good. However, the patient complained of steroid side-effects.

或

The clinical response to treatment was good; however, the patient complained of steroid side-effects.

不能写成：

The clinical response to treatment was good, however, the patient complained of steroid side-effects.

另外，however可以作为插入语放在句中：

The patient, however, complained of steroid side-effects.

2. thus和therefore一般也是副词，不能直接跟在逗号后面，要跟在句号或分号后面或者跟在and后面；它们后面一般也要加上逗号。

可以写成：

Recurrence has been reported. Therefore (Thus), the patient should be monitored closely.

Recurrence has been reported; therefore (thus), the patient should be monitored closely.

Recurrence has been reported, and, therefore (thus), the patient should be monitored closely.

Recurrence has been reported, and therefore (thus) the patient should be monitored closely.

不能写成：

Recurrence has been reported, therefore (thus), the patient should be monitored closely.

另外，therefore/thus 也可以放在句中:

The patient therefore (thus) received chemotherapy.

It is therefore (thus) likely that the adverse reaction is secondary to reduced responses to the treatment.

therefore/thus还可以接动名词作为补语：

The patient had a recurrence, therefore (thus) experiencing more pain.

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