Understanding how to use 了 (le) and when it's not present in Mandarin is really important for getting the hang of the language.

At first, beginners might think of 了 (le) just as a little word that shows something happened in the past or is finished. But it actually does a lot more than that! Using 了 (le) can change the meaning of a sentence quite a bit. Let's take a look at some examples to see how this works.

First, think about these two sentences:

• 我吃了 (wǒ chī le) means “I have eaten” or “I ate.”
• 我吃 (wǒ chī) means “I eat” or “I am eating.”

When we use 了 (le), it tells us that the eating is complete. It's like saying, “I finished eating, and that matters right now.” Without 了 (le), it sounds more like a general action that could still be happening, like “I eat” regularly.

Now, let’s look at another example:

• 他们做了作业 (tāmen zuò le zuòyè) means “They have done their homework.”
• 他们做作业 (tāmen zuò zuòyè) means “They do homework.”

In the first sentence, 了 (le) tells us they finished their homework. The homework is done! In the second one, it just says they do homework without saying if it's done or not, suggesting that they do homework often.

Let’s check out these sentences:

• 他去了商店 (tā qù le shāngdiàn) means “He has gone to the store.”
• 他去商店 (tā qù shāngdiàn) means “He goes to the store” or “He is going to the store.”

The first sentence shows that he is done going to the store and is not where he was before. The second one could mean he frequently goes to the store, without clarifying if he is there right now.

Here’s another pair:

• 我见过他 (wǒ jiàn guò tā) means “I have seen him.”
• 我见他 (wǒ jiàn tā) means “I see him” or “I am seeing him.”

In the first one, it highlights that you have seen him at some point, but it doesn't explain if it’s finished or ongoing. The second one doesn’t say whether it’s the first time seeing him or if it’s happening now.

Next, let's look at:

• 你明白了我说的话吗 (nǐ míngbái le wǒ shuō de huà ma) means “Do you understand what I said?”
• 你明白我说的话吗 (nǐ míngbái wǒ shuō de huà ma) also means “Do you understand what I said?”

The first sentence asks if you understood the specific thing that was just said, while the second one is more general—not linking to when it was said.

Next example:

• 这件事我已经告诉你了 (zhè jiàn shì wǒ yǐjīng gàosù nǐ le) means “I have already told you about this matter.”
• 这件事我已经告诉你 (zhè jiàn shì wǒ yǐjīng gàosù nǐ) means the same thing, but without the extra meaning.

The first one tells us that telling you is completed and is important for now. The second one mentions it’s been said before, but it doesn’t feel as clear.

Finally, consider this:

• 她回来了 (tā huílái le) means “She has returned.”
• 她回来 (tā huílái) means "She returns" or "She is here."

The first tells us that her return is finished and matters in the conversation. The second one sounds more like a simple statement that does not emphasize her completing the action.

From these seven examples, we can see how 了 (le)—and sometimes 过 (guò)—make a big difference in how we express time and actions in Mandarin. Getting a good grip on these details helps learners communicate better in speaking and writing. With practice, students can really improve at understanding and using these subtle differences in Mandarin.