Americans are often described as direct. That is partly true. They are direct about plans and tasks, but often soft and positive when they criticize.
"No, thank you" is a normal, polite answer. You do not need a long reason.
If you mean yes, say yes. If you refuse food once to be polite, the host will usually not offer again.
"I'll think about it" often means no.
Feedback at work and school
Criticism usually comes wrapped in positive words, sometimes called a "sandwich": praise, problem, praise.
"This is a great start. One thing you might consider is the summary. Overall, really nice work."
The middle part is the important message. Act on it.
"Interesting" or "That could work" may mean the person is not convinced.
Asking questions in class and meetings is expected and seen as interest, not disrespect.
People use light jokes and sarcasm (saying the opposite of what you mean, for example "Oh, great" when something goes wrong). If you are not sure, it is fine to ask: "Are you joking?"
Self-deprecating humor (joking about yourself) is common.
Jokes about race, religion, gender, accents or disability are offensive in most settings, including "as a joke".
Topics to avoid with people you do not know well
Money: salary, rent, how much something cost.
Age, weight and appearance: "You got fat" or "You look tired" is considered rude.
Marriage and children: "Why aren't you married?" or "When will you have kids?"
Religion and politics: many people avoid them at work. Politics can be a very divided topic.
Immigration status: you do not need to share your visa type with neighbors or coworkers.
If someone asks you a question that feels too personal, you can say with a smile: "Oh, I'd rather not say."
The US is large and varied.
Big cities in the Northeast can feel fast and less chatty.
The South and the Midwest are often known for friendly small talk and politeness ("Yes, ma'am").
The West Coast is often relaxed and informal.
Food, weather, sports and accents also change by region. These are generalizations; people are individuals.
Taking a short email as rude. Many Americans write short emails because they are busy.
Hearing "Good job, but..." and only remembering "Good job".
Commenting on someone's body as a friendly remark.