This is from a song lyric
If a picture paints a thousand words,
Then why can’t I paint you?
The words will never show the you I’ve come to know.
If a face could launch a thousand ships,
Then where am I to go?
There’s no one home but you,
You’re all that’s left me too.
And when my love for life is running dry,
You come and pour yourself on me.
First the singer says If a picture paints a thousand words.
Followed by, If a face could launch a thousand ships.
Why does the singer sing “If a face could launch a thousand ships”?
Why not, “If a face can launch a thousand ships”?
Why is can used in the past?
To start with, could can be read as a conditional. It could launch a thousand ships if the conditions are right.
Secondly, you can indeed read it as past, because it refers to a (mythological) past event.
It is a reference to the quite famous story of the Trojan War. That war, as legend has it, was waged because the prince of Troy abducted the most beautiful woman in the world (Helen).
Because of that, the Greeks assembled the biggest assembled fleet they could, rumoured to be a thousand ships, to besiege Troy. This is all very nicely described by Homer, a Greek writer.
Helen was said to be so beautiful, that her face launched a thousand ships.