Besides providing interpretation, a commentary sentence focuses closely on the text and on clarifying the meaning and significance of a text.
Commentary sentences would be helpful with respect to Act I, Scene 5, when Tybalt sees Romeo at the Capulet party. An incensed Tybalt then calls Romeo "a villain," and he says that he will not allow Romeo to be there. But his uncle, Lord Capulet, cautions him to leave Romeo alone:
He shall be endured:
What, goodman boy! I say, he shall: go to;
Am I the master here, or you? go to.
You'll not endure him! God shall mend my soul!
You'll make a mutiny among my guests! (1.5.75-79)
The commentary for the above passage could mention that this passage acts as foreshadowing for the confrontation between Tybalt and Romeo in Act III when they have a sword fight.
Lord Capulet's anger in the midst of a celebration for his daughter highlights the theme of love and hate. Also, the love that will soon be sparked between Romeo and Juliet will be set against the backdrop of hatred and family feuds, which will further illustrate this theme.
A commentary sentence is a type of sentence that you, the writer, write that comments on facts presented in a previous sentence or earlier in that paragraph. The commentary sentence allows you, the writer, to put in some of your opinion, analysis, and interpretation of facts.
From the play "Romeo and Juliet" you could use a specific line to write your commentary about. Let's use this line:
"Henceforth I shall never be Romeo...My name, dear saint, is hateful to myself / Because it is an enemy to thee"
Romeo speaks that line in Act 2 Scene 2. Your commentary sentence could be the following: "The quote illustrates to the reader that Romeo is not close to his family, because it shows Romeo's ability and desire to casually disassociate himself from the Montague family name."
If you don't want to use a single line of text, you could use a dialogue sequence between two characters, a monologue, or a soliloquy. Use Friar Laurence's soliloquy at the start of Act 2 Scene 3. Your commentary sentence could be this: "The Friar's opening soliloquy is important to the play because it shows to the audience that the friar is knowledgeable about herbs, potions, and the duality of human nature. It is also foreshadowing his help to Juliet and his desire to heal the feud between Capulets and Montagues."