swear (PROMISE)  verb
 to state or promise that you are telling the truth or that you will do something or behave in a
 particular way
 I don't know anything about what happened, I swear. [I]
 You might find it difficult to believe, but I swear (that) the guy just came up to me and
 gave me the money. [+ (that) clause]
 New gang members must swear to obey the gang leaders at all times. [+ to infinitive]
 In some countries, witnesses in court have to swear on the Bible. [I]
 I swore an oath to tell the truth, the whole truth and nothing but the truth. [T]

 When someone is sworn in, they make a formal promise to be honest or loyal, either because they
 are in a law court or because they are starting a new official job.
 The next witness was sworn in. [M]
 William Jefferson Clinton was sworn in as the 42nd President of the United States of
 America. [M]
 A few of us knew what was going to happen, but we were sworn to secrecy (=we were
 forced to promise that we would keep it a secret). [T]
 INFORMAL I think his birthday is on the 5th, but I wouldn't/couldn't swear to it (=I am not
 completely certain about it). [I]

 ESPECIALLY BRITISH INFORMAL She swore blind (=emphasized that it was true) (that) she
 didn't know what had happened to the money.

 INFORMAL My dad swears by (=believes in the effectiveness of) these vitamin pills.