or:  a colleague suddenly arrives at work much earlier than usual

you say to him: 'You're early today!'

so, you're in the office earlier than usual today!

that's unusual for you. I'm surprised to see you here so early

'it's a secret! Don't tell anyone I told you!'

here we see another function of 'yes,' namely in a command sentence

it makes it clear that the command is very serious and urgent

under no circumstances should you tell anyone that I told you this

or: 'It's really cold outside. Dress warmly!'

here, the command is also reinforced: 'You must – absolutely – dress warmly!' 

it's very important that you dress warmly

another example: 'Drive carefully! It's below zero and the roads are slippery!'

so: Please drive, absolutely, very carefully

'I'd love to come with you, but I don't have time'

'ja' sometimes occurs in conjunction with the word 'but'

'ja ... aber' connects two sentences

the first sentence contains a statement, emphasized by the 'ja'

in this example: I would like to come along. I would really like to come along

and in the second sentence, a qualification is added

I explain why what I said in the first sentence doesn't work, isn't true, and so on

here's another example: 'The dress is beautiful, but much too expensive'

so, I think the dress is really beautiful, but it's too expensive

or: 'I'd love to go on vacation with you, but three weeks is too long for me'

'it's supposed to snow tomorrow. Yes?' Here, 'JA' is a question in its own right

it means something like 'Really?', but it's a bit weaker

if someone says something you find surprising ...

or have slight doubts about

then you can respond with 'JA'. It's supposed to snow tomorrow'

I wouldn't have expected that. Is that really true?

another example: 'Lisa got the job.' 'Yes? That was quick'

so: she really got the job already?

I'm surprised it was so quick

or: 'The new (female) boss is really nice.' 'Yes? Great!'

there's a hint of surprise here, too

oh, the boss is really nice. Great!

this 'JA' expresses surprise, but sometimes it also hints at doubt

it really depends on the situation and what you say afterward

here's another example: 'That dress looks really good on you'

Really! I thought you didn't like yellow dresses

there's a hint of doubt here

are you really serious?

do you really think the dress is beautiful? 

that surprises me