Tim Bowen will never let you down when it comes to phrasal verbs.
‘The icy weather that has gripped Europe shows no sign of letting up’ according to a news report. Here let up means to ease up or to reduce in intensity. Usually used in the negative, let up can also mean to put less effort into something, especially something you have been doing with great determination or enthusiasm, as in ‘She has never let up in her efforts to prove that her brother is innocent’.
‘She didn’t know what she was letting herself in for when she married John’, means that she put herself in a situation that would be more difficult than she expected. If you let someone in on a secret or a plan, you make them aware of it, as in ‘They were planning something, but they wouldn’t let me in on it’. If you let on, on the other hand, you talk about something that is intended to be secret, as in ‘Don’t let on that I told you’.
Often used in the passive voice, let off can mean to give someone little or no punishment for something wrong that they have done, as in ‘The police pulled me over for speeding but I was let off with a warning’.
To let down means to make someone disappointed by not doing something that they are expecting you to do, as in ‘Everyone else arrived on time but, as usual, Tony let the rest of the team down by turning up late’. Again, often used in the passive voice, let down can also be used to make someone or something less effective, as in ‘The new model is an impressive car but it is let down by its excessively high fuel consumption’.