Today, I’d like to explain another short expression that we often use in English: “in case”. We also can say “just in case” in some situations. We use this expression when we are talking about doing something as a preparation for a possible negative future situation. For example:
I brought my umbrella today in case it rains.
I brought a sandwich for you in case you were hungry.
I brought two pens with me to class in case one of them runs out of ink.
You should take your credit card with you on your trip in case you spend all your cash.
As you can see, we often use the words “bring” or “take” in these sentences. In these sentences the person brings something with them (the umbrella, the sandwich, the extra pen, the credit card) in order to prepare for a negative thing that COULD happen (it rains, the friend is hungry, the first pens runs out of ink, the person spends all their cash). It is important to note that the negative event is only a POSSIBILITY, and the person realizes that it might not happen.
We can also use “just in case” with these sentences, but the structure will change a little. For example:
It might rain today, so I brought an umbrella just in case.
I thought you might be hungry, so I brought you a sandwich just in case.
My first pen might run out of ink, so I brought a second one just in case.
You might spend all your cash on your trip, so you should take your credit card just in case.
We can also use “in case” or “just in case” when talking about preparing for a situation which is not negative but is unlikely or unexpected. For example:
I know the cherry blossoms probably aren’t going to be in bloom today, but I brought my camera in case they were.
I know the cherry blossoms probably aren’t going to be in bloom today, but I brought my camera just in case.
I hope this is clear to everyone. These expressions can be difficult to use so, as I’ve said before, please study the full sentences and memorize them, so you will be able to use them in conversation.
English Help Online's Blog 
gs804 Said:
on March 11, 2010 at 2:40 pm
Thank you for your useful everyday entries!
Now, I have a question about the second “sandwich” example.
Only the second example has past tense after the phrasal verb for today, so could you tell me what this difference of the tense is?
Does this past tense show the less possibility?
englishhelponline Said:
on March 12, 2010 at 3:47 pm
Hi there.
Thank you for your reading my blog and for your kind comments.
Your question is a very good one and I had to think about it, but I know what you mean now.
In most cases, we use “in case” for a possible future situation as in the first example, “in case it rains”. This means if it rains in the future.
However, when we say “I brought you a sandwich in case you were hungry”, we have to use the past tense “were” because by the time he or she brings the sandwich to the person, they might ALREADY be hungry and the hunger might have started in the past.
Is that clear? I hope so. If not, please let me know.
Mike
gs804 Said:
on March 15, 2010 at 12:14 pm
Thank you for your reply to my question. And I’m sorry to be late to respond to your answer.
Now it becomes very clear and I’m glad to know these little points and differences have each reason.
And the entry about difference among “can you”, “could you”, “would you” is also very useful and easy to undrstand, especially the example of “Would you marry me?” is so good. Thank you!