What is a curved mark over vowel used to show that it has short sound?

The curved mark over a vowel used to show that it has a short sound is called a breve. It looks like this: ˘

Here are some examples:

* ă as in "cat"

* ě as in "bed"

* ĭ as in "big"

* ŏ as in "hot"

* ŭ as in "cup"

It's important to note that the breve is not always used in English writing. It is more common in dictionaries and phonics instruction to indicate the short vowel sound.