1.testify的基本意思是“为…作证”“证明”,指通过某人证明某事的真实性。testify主要用于法庭或其他正式场合,一般比较郑重。
2.testify可用作及物动词,常接that从句作宾语,意思是“证明说”。
3.testify against意思是“作对…不利的证明”; testify in favor of意思是“作对…有利的证明”; testify for意思是“证明…无罪”。

testify against(v.+prep.)
作对…不利的证人〔词〕 be a witness, usually in court; prove the guilt of (sb)
▲testify against sb
None of the victims would appear in court to testify against him.受害者中无人愿意出庭作指控他的证人。
He hated to testify against a friend.他不愿出庭作对朋友不利的证词。

testify for(v.+prep.)
证明某人无罪 be a witness, usually in the court, in favour of (sb)
▲testify for sb
Only members of his family have testified for him.只有他的家人证明他无罪。

testify in favor of(v.+prep.+n.+prep.)
作对…有利的证词 give evidence in favor of sb in a court of law
▲testify in favor of sb
He will testify in favor of the defendant.他愿意出庭作对被告有利的证词。
He offered to testify in favor of the accused.他主动出庭作对被告有利的证词。

testify to(v.+prep.)
为…作证,证明 bear witness to
〔说明〕 testify to常不用于进行体。
▲testify to sth
I can testify to his honesty.我可以为他的诚实作证。
I can testify to this man's veracity and good character.我可以作证,此人诚实可靠,品德良好。
I can testify to his innocence.我可以证明他无罪。
The teacher testified to the pupil's ability.老师证实了学生的能力。
Her tears testified to her grief.她的眼泪证明了她的悲伤。

用作不及物动词
▲S+～(+A)
The witnesses testify in the law court.证人在法庭上作证。
用作及物动词
▲S+～+that-clause
One witness testified that he'd seen the robbery.有一个证人作证说他看见了这次抢劫。
The witness testified that the larger car crowded the smaller one into the ditch.这个现场目击者作证说是那辆大车把小车挤到沟里的。
He testified that the defendant was innocent.他作证说这个被告是无辜的。

testify, prove
这组词都有“证明”的意思。其区别是:
1.testify是指证明某事的真实性; prove是指证明某人或某物具有的某种品质特性。
2.testify指通过某人证明; prove指通过事实证明。
3.testify用于法庭或其他正式场合,比较郑重。
4.用作及物动词时, testify后可接从句作简单宾语; prove后可接复合宾语。用作不及物动词时, testify后常接against, for〔on〕 behalf of或to等词。

下面两句意思相同:
Her nervous behavior testified to her guilt.
Her nervous behavior testified that she was
guilty.
她的紧张举止说明自己有罪。