Spanish language Q&A Forum

A quiz question says: "La palabra víspera tiene el tilde en la i"

Not relevant to the lesson, but I thought a tilde was the squiggle over the n (~), not the tick mark? Can it mean both?

Are por qué no/ mejor no only used with vamos? Or are these phrases more commonly used for all commands (not just ir)? 

For the road signs, are they third person singular for affirmative commands, and third person plural for negative commands? Or are both singular and plural third person used equally for either negative or affirmative commands?

Una vez alcanzado su tamaño máximo, la larva se prepara para.....

Should this be una vez alcanzada as it refers to the feminine noun larva ?

Gracias