be, look, seem | become | remain, stay | keep sb
The machine should help to keep the pilot alert.

very | fully | immediately, instantly, suddenly
There was a noise outside and he was suddenly alert.

always, constantly | mentally
He was as mentally alert as a man half his age.

to
Climbers need to be alert to possible dangers.

red
His sudden disappearance triggered a red alert among his friends.

nationwide | bomb, fire, flood, pollution, security
A security alert was issued after four men escaped from the prison.

issue, put out, raise, sound
They rang the church bells to sound the alert and the villagers then fled.

spark, trigger | call off
The alert was called off when it was found that the bomb was not live.

go out
A nationwide alert went out for three escaped prisoners.

on the ~ | ~ for
You should always be on the alert for anyone who looks suspicious.

be on full alert
The security forces are now on full alert.

keep/put/place sb on full alert
Thousands of police were put on full alert at all main roads leading to the city.

a state of alert
The army was yesterday placed on a state of alert in case of more riots.