old speech

Doubt thou the stars are fire
Doubt that the sun doth move
Doubt truth to be a liar
But never doubt I love

-Shakespeare in Hamlet

In modern English this mean that:
You can doubt that the starts are made of fire
Doubt that the sun moves
Doubt that truth is a lie
But you can not question my love.

 He is trying to say that you can doubt all these proven things that have evidence behind them but you can not doubt his love.
The syntax of the quote is not that different to what we would use now although we have added words in.
The word 'doth' is no longer used in modern English we use the word 'do' instead. Or in this case we would say doubt that the sun does move.
Shakespeare also uses never doubt I love where as we would say my love.