About the spelling of this word, it actually has many spellings: 其实这个字有很多的拼法
But it doesn't really matter because it's very casual and colloquial: 因为这是口语化的表达，在正规的工作邮件里面还是不要用Yeah了。
It's pretty much the same as "yeah" with a "p" at the end: 跟Yeah很像, 就是加个“p”在后面。Sounds kind of nice and a little more funny.
It's very similar. It's also informal and casual: 它的用法跟yeah也很像。
比如说: Let's grab dinner tonight together. Are you free?
在工作场合也可以用，比如你可以跟同事说: Adam, can we do some recordings later this afternoon?
It's the best way to reply if somebody asked you "Can you do something?" or "Did you do something?" I'd propably say "yup" before "yeah". I wouldn't say "Yeah, I did": 这是一种友好的表达，表明你们是相互认识的。
The tone of "yeah" can sometimes sound impatient, but "yup" always sounds positive, like a friendly dog: “Yeah”的语气如果稍有不慎就会给人以不友好或不耐烦的感觉。而“yup”则给人一种更积极更坚定的感觉。
可以拼写成 yup，或者 yep.
It's everything about "yup" but cuter: 很有旋律感像唱歌一样的yuppers，用法跟yup很相像。像是美国牛仔经常会使用 "yuppers"。是非常可爱的一种表达。
It's not really "Yes, sir" but more in a friendly ironic way. If you are saying to a person much above you, you would say "Yes, sir." But this case is with friends: "Yessir"来源于“是，长官”，但在长期逐渐演化变体中，形成了更口语化也带有友好反讽自嘲意味的“yessir"。
The only problem is sometimes you want to say "Yessir" to a girl, so there's also "Yes'm" short for "Yes, ma'am", which comes from the southern part of America: 所以“Yessir”多用于男子之间，而对女士则会用"Yes'm"。
It also sounds very cute: Okey-dokey 很有二次元卡通的意味。
Another way to say "okay"
For example, in the office I often answer "Adam, can you finish something by 2 o'clock?" with "Okey-dokey, no problem"
When people ask you to do something
If you are tired of using the same word again and again, you can switch it up and say "rightio". British people tend to say "righto", but it's the same thing. It tends to be a Commonwealth term, not so much American: 总之不管是rightio还是righto都是常见于英联邦国家，而在美国反而不太常见。
Americans love using "totally". 美国人的口头禅绝不能漏掉了这个词。
Exactly, totally, and absolutely are three of the most common words you'll hear Americans say, since they have a tendency to exaggerate: Exactly，totally，absolutely简直就是美国人最喜欢用词的“top three”前三甲。美国人喜欢“over the top”，喜欢夸张，特别喜欢totally。
比如问一个电影好看么，"Did you like the movie?" 回答也是"Totally".
"It's totally worth it."
"You should totally go."
"It was totally awesome."
如果你问他们 "Do you like China?" 回答也是，"Absolutely," "Totally".
So many times I've seen Chinese friends and I've asked them, "Do you like this?" I can see their face. They really like it but they can only say "yes". They know it's not enough and they're trying to say something more.
This one is dead simple. It isn't so much a word as it is a sound. It might sound unenthusiastic, but it all depends on your tone of voice and where you add stress: 这实际上也不是一个词，只是发出一个声音，听起来或许有一些不情愿，没什么热情，但是尝试变变声调，就会有不同的感觉。
Oprah, one of the most famous American talk show hosts, loves using this word: 这还是黑人脱口秀女王奥普拉很喜欢说的一个词哦。
I'm thinking about an old singer named Ray Charles, who sang for a Pepsi ad: "You got the right one baby. Uh-huh!"