Here's the original by Lope de Vega:

Soneto de repente

Un soneto me manda hacer Violante,
que en mi vida me he visto en tanto aprieto;
catorce versos dicen que es soneto,
burla burlando van los tres delante.

Yo pensé que no hallara consonante
y estoy a la mitad de otro cuarteto,
mas si me veo en el primer terceto,
no hay cosa en los cuartetos que me espante.

Por el primer terceto voy entrando,
y parece que entré con pie derecho
pues fin con este verso le voy dando.

Ya estoy en el segundo y aun sospecho
que voy los trece versos acabando:
contad si son catorce y está hecho.

And here's my translation, copyright John Cowan 2001:

My friend asked me to make for her a sonnet;
I've never found myself in such a fix.
Fourteen lines, they say, make up a sonnet;
I'll write the next three parts with clever tricks.

I was not born beneath a rhyming planet,
Yet halfway through this poem I'm still here.
And if I catch myself a final couplet,
There's nothing in the quatrains I need fear.

The third verse, as it seems, I'm now beginning;
It's likely that I'll make it to the end
Of this game that I am so slowly winning,
This poem that I'm making for my friend.

My thirteenth line, I see, I'm almost ending;
Do you count fourteen? -- if not, well, 'tis past mending.