Spanish verbs ending in -gar replace the -g- with a -gu- in El Presente de Subjuntivo. This is to preserve the sound from the infinitive.
Here is the conjugation of pagar (to pay) affected by this spelling change:
| yo |
pague |
| tú |
pagues |
| él / ella / Ud. |
pague |
| nosotros |
paguemos |
| vosotros |
paguéis |
| ellos / ellas / Uds. |
paguen |
Read and listen to these examples:
No descansaré hasta que pague la deuda.I won't rest until I pay my debt.
No voy a aceptar que pagues por todo.I am not going to allow you to pay for everything.
Deja que él pague las cervezas.Let him pay for the beers.
No tendremos vacaciones hasta que paguemos la hipoteca.We won't have any holidays until we pay the mortgage.
Es justo que vosotros paguéis la mitad.It is fair that you pay half.
Paguen en esa ventanilla, por favor.Pay at that counter, please. ("you" formal plural)
There are other useful verbs that have this same g > gu spelling change, for example: apagar (to turn off), entregar (to deliver), jugar (to play)
Here are more examples with these verbs:
No apagues la luz del salón, por favor.Please don't turn off the light in the lounge.
Cuando me entreguen el contrato seré feliz.I'll be happy when they give me the contract.
Juguemos un rato a las cartas.Let's play cards for a bit.
See the List of -gar verbs change in El Presente de Subjuntivo
No apagues la luz del salón, por favor.Please don't turn off the light in the lounge. No tendremos vacaciones hasta que paguemos la hipoteca.We won't have any holidays until we pay the mortgage. Juguemos un rato a las cartas.Let's play cards for a bit. Es justo que vosotros paguéis la mitad.It is fair that you pay half. Cuando me entreguen el contrato seré feliz.I'll be happy when they give me the contract. No descansaré hasta que pague la deuda.I won't rest until I pay my debt. No voy a aceptar que pagues por todo.I am not going to allow you to pay for everything. Deja que él pague las cervezas.Let him pay for the beers. Paguen en esa ventanilla, por favor.Pay at that counter, please. ("you" formal plural) 
