What is the difference between “ran out of” and “have/has run out”?
Ex: We ran out of milk
We have run out of sugar.
Thank you.
The first is saying it happened in the past, at some time. With ‘have’ it’s saying that it’s only just happen or has just been noticed. Typically shortened to ‘We’ve…’
‘We ran out of milk’ means that sometime in the past, ‘yesterday’ for example, we had no milk because we had used up all the milk.
So you can say: Yesterday we ran out of milk.There was no milk to put in our coffee. I went to the shop to buy some more milk. Today there is enough milk for our coffee.
‘We have run out of sugar’ means that now we cannot make a cake or put sugar in our coffee because we have used up all the sugar.
So you can say: Oh no! We have run out of sugar. There is no sugar to put in our coffee.
I will go to the shop to buy some more.
By the way, ‘used up’ means that we have used something (for example: sugar) until there is now none of it left. When it is used up, there is none of it. The sugar container is empy! ‘Used up’ is usually followed by ‘all’, as in the sentences above.
It is true. In speech we almost always shorten “we have” to “we’ve”. Not doing this in written English makes the construction clearer.
I just have a little something extra to add here… presuming that you are saying it at the time you notice you have run out of something, then it depends what kind of English you speak.
If you speak American English, when you run out of milk and you announce it to someone (e.g. a family member who you would like to send to buy some more milk) you would probably use the simple past tense, “We ran out of milk”.
In British and South African English however, it’s more common to use the present perfect tense, “We have run out of milk”. We use the present perfect to link the past to the present. i.e. We ran out of milk in the past and now there is none (to emphasise the effect the past has had on the present situation).
People from the US generally tend to use the present perfect tense less than people from the UK.
Simple example using a common everyday expression:
US: I already ate.
UK: I have already eaten.
So, it’s somewhat down to what kind of English you speak or prefer.
I personally think British English is clearer in meaning. e.g. I ran out of milk yesterday. I have run out of milk. You can see that the first sentence is past and the second sentence is what has happened right now. 
Anyway, hope this adds to the explanations above and helps you. 