Learn Spanish (Latin America): Acepto el proyecto siempre y cuando modifiquemos la tarifa. - I accept the project as long as we modify the fee.

The phrase siempre y cuando means "as long as" or "provided that". It sets up a strict condition that hasn't happened yet—the acceptance of the project depends entirely on modifying the fee. Phrases that set up these strict, hypothetical conditions always force the verb that follows into the subjunctive mood.

Why is modifiquemos spelled with a qu instead of a c?

The verb is modificar. In Spanish, if you just changed the a to an e to form the subjunctive (modificemos), the c would suddenly sound like an "s" because it's followed by an e. To keep the hard "k" sound of the original verb, we change the c to a qu. You will see this spelling change in all verbs ending in -car.

Could I use siempre que or con tal de que instead of siempre y cuando in this sentence?

Yes, absolutely! Siempre que, siempre y cuando, and con tal de que all mean "as long as" or "provided that", and they all trigger the subjunctive. Siempre y cuando sounds just a little bit more emphatic, but you can swap them without changing the core meaning of the sentence.

I have seen used for things like taxi rides. Does it also apply to business projects?