Word sets like this are among the most challenging aspects to good grammar, because it is very easy to get them mixed up. It can be difficult to remember the difference between ever so often and every so often, for instance, since there is only a difference of a single letter. With sing, sang, sung, it may not always be appropriate to “play it by ear.”
Sing is generally the “present tense” version of the word, in addition to some of the different conjugations based on the subject of the sentence. Some examples of proper use of “sing” and its variants include the following:
I sing in the shower.
He smiles when he sings.
They are singing in the courtyard.
Sang is the “simple” past tense version of the word. It is a verb that can be used on its own to indicating that the singing took place some time in the past. Some examples of “sang” would then include the following:
You sang the national anthem.
We sang as loudly as we could.
She walked and sang at the same time.
Sung is the past participle version of the word. It is a verb that, unlike sang, cannot be used on its own. Instead, it must be accompanied by a “helping” verb (usually “to have” or “to be” and their variants). It can used as the past tense, as well as present perfect and other tenses. Without going too far into the complexities of grammar, some proper uses of “sung” would include the following:
I would have sung more clearly if I knew the words.
The chorus was sung by the entire group.
Jimmy has sung at the venue many times before.
Knowing whether to use sing, sang, or sung can be challenging for even the most seasoned of English experts. Just as people still confuse biannual and semiannual from time to time, it’s understandable to mistake sang for sung sometimes too. The key is to make the appropriate correction when needed and, better yet, look up the right word before you write it in the first place.