These words and phrases can be English speaking mistakes because they might be seen as rude, so you'll want to avoid them.
Of course
The misunderstanding with "of course" is that English learners seem to think that it always means "definitely" or "certainly", but it actually means "obviously". This can be a problem and make your response with "of course" seem rude in situations when someone is asking you a question about something that they don't know.
If someone asks you a question because they don't know something and you reply by saying "obviously", it is rude because it seems like the person who asked the question is stupid. Look at the example below, where you can see that replying with "of course" or "obviously" is rude:
Person A: "Does your wife like seafood?"
Person B: "Of course (obviously)."
In the example above, Person B responds by saying "of course" but it means "obviously", so it is making Person A feel stupid for asking a question with an obvious answer. It seems like Person B is saying that it was a dumb question, but it's not, especially if Person A will cook dinner for Person B and their wife.
Here is another example where it is rude to reply with "of course":
Person A: "Is the store open on Sunday?"
Person B: "Of course (obviously)."
Again, in the example above, Person B is saying "obviously the store is open on Sunday", which sounds like: "Don't you know that? Everyone knows that!" and makes Person A feel stupid.
The one situation where it's okay to say "of course" is when someone is asking you for a favor. For example, if someone asks you: "can I use your washroom?" you can say "of course" and it will not be rude because you are accepting their request for a favor (which is a kind thing to do). "Of course" is a rude response when someone is asking you about something they don't know.
The example below is more polite:
Person A: "Is the store open on Sunday?"
Person B: "Absolutely/Definitely/Certainly/Yes"
What?
In English, it's considered rude to ask someone to repeat what they said by saying "what?" It sounds annoyed and seems like "what are you talking about??" In this case, it's much more polite to say "pardon?" or "sorry?" with an upward intonation in your voice, like a question.
The example below is more polite:
Person A: It's hot in here.
Person B: Pardon? / Sorry?
Person A: I said it's hot in here.
Wait
If you say only a present base form verb, like "go" or "sit" without any subject in front of it, it means you are commanding the listener to do this action.
For example, if you have a dog, you might say "sit" because you are commanding the dog to sit. This is called the imperative form. If you say "wait" to someone, it will sound like you are ordering them to wait, like a dog, and they will probably not like this. Instead, it's better to say "just a moment (please)" or "just a second (please)".
You can add "please" to the imperative form and say "please wait" but it's still better and more common to use the phrases above instead.
The example below is more polite:
Person A: Do you have an appointment available on Tuesday around 3pm?
Person B: Just a moment/second (please). I'll check for you.
Teacher
Don't call your teacher "teacher" in Canada and United States. I cannot speak for sure about other English-speaking countries but I know that students in Canada and United States never call their teachers "teacher", so if you call your teacher "teacher" in these countries, they might think you don't know their name or don't care what their name is.
I know that this comes from translation and the idea that "teacher" is a respectful title in your country is nice, but it will not seem that way in English. Many English teachers have heard "teacher" a million times so they are used to it, but many do not like it. If you are wondering what to say to your teacher, use the guide below, and imagine your teacher's name is Melissa Smith:
Elementary and High School: Mr. / Mrs. / Ms. (family name) = Mrs. Smith
University: Professor (family name) = Professor Smith
Adult Classes: First name only = Melissa
Girls
Often the term "guys" is used for men, but the term "girls" shouldn't be used for women. I realize that some women might not mind if someone calls them a girl or a group of women "girls" but some will not like it.
The reason is that "girls" seems like females who are young and immature, while "women" is a mature and independent person. Therefore, only use "girls" for teenagers or younger. You can also use the term "young women" for late teens and early 20s.
I want...
When someone is serving you, telling them "I want (something)" sounds demanding, like they are your servant, but serving you and being a servant are not the same thing, so it's important to show humility and respect when telling them what you want. Instead, it's better to use "can I get...?" or "I'll go with...".
The example below is more polite:
Server: What can I get you?
Customer: I'll go with a hamburger / Can I get a glass of wine?
Strange Person
The phrase "a strange person" means someone who is strange, and strange means "weird". It is considered a negative characteristic, so if you say that a man or woman is a "strange person", it is criticizing that person. People that you know can also be strange people.
On the other hand, "a stranger" is anyone that you don't know. A stranger can be normal or even very kind, it just means that you don't know them.
The example below is more polite:
I lost my ring at the park but some strangers helped me find it.
If you want to learn 200 incorrect phrases that English learners often say and how to correct them, check out my book "Stop Saying That!"
Practice
Change the rude sentences below to make them more polite:
My new coworker is a girl from India.
What? I didn't hear you, it's too loud in here.
I want a room with a king sized bed.
Wait. I have to ask my supervisor.
A strange person helped me carry my suitcases down the stairs at the subway station.
Teacher, is this sentence correct? (your teacher's name is David White)
Of course the library has photocopy machines. (the question was "does the library have any photocopy machines?)
Answers
My new coworker is a woman from India.
Pardon? / Sorry? I didn't hear you, it's too loud in here.
Can I get a room with a king sized bed? / I'll go with a room with a king-sized bed
Just a moment/second (please). I have to ask my supervisor.
A stranger helped me carry my suitcases down the stairs at the subway station.
David, is this sentence correct? / Mr. White, is this sentence correct?
Yes (the library has photocopy machines.) / Definitely/Absolutely/Certainly (the library has photocopy machines).