The verb "to let" is an irregular verb that is one of the most frequently used words in English, thanks to its role in expressing everything from simple permission to complex emotional release.
1. The Core Grammar Rule: The Bare Infinitive
The most critical grammar point for the "permission" meaning of let is that it is always followed by an object and the bare infinitive. The bare infinitive is the base form of the verb without the word "to."
| Incorrect (Full Infinitive) | Correct (Bare Infinitive) |
| She let me to borrow her car. | She let me borrow her car. |
| Don't let the problem to affect you. | Don't let the problem affect you. |
| He didn't let us to go early. | He didn't let us go early. |
Important Exception: The Passive Voice
The verb let is rarely used in the passive voice when it means "to allow." Instead, English speakers substitute it with "to be allowed to" or "to be permitted to."
- Active: The guard let the protestors through the gate.
- Passive (Correct): The protestors were allowed to pass through the gate.
- Passive (Incorrect): The protestors were let through the gate.
2. Phrasal Verbs and Idioms of Release
The greatest complexity of "to let" lies in its numerous phrasal verbs and idioms, almost all of which carry a sense of release, disclosure, or relaxation.
| Phrasal Verb / Idiom | Meaning | Example |
| Let down | To disappoint someone; to fail to keep a promise. | I was counting on you for help; you really let me down. |
| Let off | To excuse someone from punishment or duty. | The police officer let him off with a warning for speeding. |
| Let up | To decrease in intensity, stop, or slacken (often used for weather or pressure). | The constant pressure from work never seems to let up. |
| Let out | To make a sound; to release from confinement; to enlarge clothing. | She let out a huge sigh of relief when the news was over. |
| Let on | To reveal a secret or information (often used in the negative). | Don't let on that you know about the surprise party! |
| Let go (of) | To release one's physical or emotional hold on something. | You need to let go of the past and focus on the future. |
| Let the cat out of the bag | To accidentally reveal a secret. | I accidentally let the cat out of the bag about their engagement. |
| Let one's hair down | To relax completely and enjoy oneself. | After the exam, the students were ready to let their hair down. |
| Let be | To leave something alone; to stop interfering. | I'm trying to concentrate, so just let me be for a few minutes. |
3. The Special Imperative: "Let's"
The contraction "Let's" (short for Let us) is grammatically distinct because it is an imperative construction used to make a suggestion to a group that includes the speaker.
- Function: It is the only way to form a first-person plural imperative in modern English.
- Negative Form: The common negative is "Let's not..."
- Correct: Let's not make a hasty decision.
- Formal: Let us not deceive ourselves.
4. Property and Business Usage (Primarily British)
As previously noted, "to let" is a formal, commercial verb in the UK and Commonwealth countries meaning to lease or rent out property.
- Landlord's Action: A landlord lets a property.
- Tenant's Action: A tenant rents a property.
- Signage: You will see signs on buildings that simply say "TO LET" (available for rent).