In English, you should only use "let's" when you yourself will also do the action. For example, you can say to your friends, "Let's go to the beach!" meaning I want to go to the beach with you.
However, if you won't be doing the action but you want to encourage others to do something, you shouldn't use "let's." For those cases, you can simply say:
・You guys go to the beach!
・Why don't you go to the beach?
In the context of school, teachers rarely use "let's," because they don't usually participate in the exercises. Instead they will say to their students:
・(Please) read page 5 aloud.
・(I want you to) form groups of 6.
*The same is true for advertising - ads in English will almost never use the word "let's," but instead will say "Try this" or "Discover that" or "Feel this" etc.
This is the imperative mood (命令法), which might sound rude in Japanese but is perfectly natural in English. It will only sound rude if you make your voice loud or angry.
2. Meet vs. see
If you say to a native speaker, "I met my parents last weekend," they might be confused. Here's why:
You might have learned that "meet" means "会う" in Japanese. This is true, but in most cases we actually use the verb "to see" for "会う." Let me explain. "To meet" is mainly used for the first time you ever meet someone in your life. For example:
・I met my best friend in the first grade.
For people you have already met for the first time, when you do things with them you should use the verb "to see." For example:
・I saw my best friend yesterday. We each ate six bowl of ramen.
Another great way to say this is to use the phrasal verb "meet up with," like this:
・I met up with my best friend yesterday. We each ate six bowls of ramen.
3. School grades
Speaking of the first grade...Since the school system and grade levels differ between Japan and other countries, it can be confusing for non-Japanese people when we hear what grade you are in, or your child is in. I'm from the U.S., so I'll tell you about our system.
There are 13 public school grades in the U.S.: K-12. In elementary school, we have grades kindergarten through 5th or 6th grade, depending on the state. That means that when we are 5 years old (usually), we start in kindergarten. At 6 years old, we move to 1st grade, at 7 years old we move to 2nd grade, and so on.
Middle school (also known as junior high school) is grades 6-8 or 7-8. So we start middle school at 11 or 12 years old, depending on the state.
Finally, high school is always four years, and grades 9 through 12. We enter at 14 years old. These grades are also known as freshman year (9th grade), sophomore year (10th grade), junior year (11th grade), and senior year (12th grade).
Here is a conversion chart from Japanese school grades to U.S. school grades:
幼稚園・kindergarten (this is included in elementary school in the U.S.)
１年・7th grade (She is in 7th grade, in the first year of Japanese middle school.)
３年・9th grade (She is in 9th grade, in the last year of Japanese middle school.)
１年・10th grade (She is in 10th grade, in the first year of Japanese high school.)
4. Know vs. find out vs. hear vs. get to know
The verb "know" is actually kind of limited. We can use it to tell something that we currently are aware of, such as:
・I know that poop-shaped goods are popular in Japan. (Hehe, they are so cute!)
If you want to talk about the past, about when you first became aware of something, you need a different verb: "to find out," or sometimes "to hear." For example:
・Yesterday I found out that pigs run faster than chickens. (It's true!)
・The other day I heard that a hippo is more likely to kill you than a shark. (Also true!)
Finally, one more useful verb to know (・ᴗ-)✧ is "get to know." This is used for the process (over time) of understanding someone or something. It's often used for people - but can be used for things as well. Here's an example:
・I got to know my husband while we were stranded in an elevator for two days. (Not true.) （＾ｖ＾）
That's all for now. Let me know in the comments if you have any questions!