Why does Romeo seem to love Rosaline and then very quickly love Juliet? (Romeo and Juliet, Shakespeare, play taught at GCSE)

This is a really good question that's answer is central to the the plot of Romeo and Juliet. I agree that it does seem surprising that after spending a great deal of the first four acts of the play exclaiming his love for Rosaline.
The answer to this question lies in the language. By paying close attention to to the ways in which Romeo speaks about the two ladies we are able to understand that his affections for them are very different.
Here I would pick two of Romeo's speeches from the play, one where he describes his feelings for Rosaline and the other where he speaks about Juliet in Act 1, Scene 5.
In this example I will suggest a comparison of Romeo's description of his feelings of Rosaline in Act 1, Scene 2 and his feelings for Juliet in Act 1, Scene 5.
By pasting the two speeches side by side on the whiteboard, I will encourage the student to look for the adjectives that Romeo uses to describe the two ladies. I will underline the words the student draws attention to, and ask them to give me a sentence or two explanation as to why they have made that choice. If the student is stuck, I will draw attention to the following language with the highlighter function and explain my choices as I do so:
Act 1, Scene 2 where Romeo describes the way in which his feelings for Rosaline make him 'mad' and bring him pain. He is 'torment[ed]' by his feelings for her and his association of her with the the sun and heat indicate that his feelings are passionate and making him have lustful and sinful thoughts.
I will contrast this by drawing attention to Romeo's description of Juliet when he first meets her in Act 1, Scene 5. Romeo describes himself as a pilgrim who worships Juliet. Instead of causing him pain and making him wish to sin, Romeo's is 'devoted' to Juliet and he worships her as though she were holy.
The ways in which Romeo's feelings for Juliet differ from his feelings for Rosaline earlier in the play are confirmed in Act 2, Scene 2. Romeo asks Friar Laurence to bless his marriage to Juliet. Here Friar Laurence scolds Romeo for his fickleness and for crying over a lady who he didn't really love when he questions:
'How much salt water thrown away in waste,To season love, that of it doth not taste!'
By using this example, I will show how close attention to the language can hold the answers to many questions about the play. By looking closely, and providing the student with strategies for breaking down the complex sentences into small, bite-size parts, I will demonstrate a method they can employ in the close-reading section of their exam so that whatever extract is thrown at them, they have the key-skills to both understand what is happening in the scene, and to provide a cracking answer!