Is it grammatically correct -be singing?

"Be singing" is grammatically correct, but it's not a complete sentence. It's a present participle phrase.

Here's how you can use it:

* As part of a sentence: "I will be singing at the concert tomorrow."

* In a sentence with a linking verb: "They are be singing in the choir."

The phrase "be singing" describes an ongoing action. To make it a complete sentence, you'll need a subject and a verb:

* Subject + Verb + "be singing": "The birds are be singing in the trees."

Let me know if you'd like to see more examples or have any other questions!