The Subjunctive tense suggests a situation that is hypothethetical, unreal or that is not concrete. It is used to express likes, wants, desires, needs, and doubt. As a result, the independent subjunctive verb causes the dependent verb to have a tense change, making the -ar ending the -er/ir ending switch.
Let's take the verb "tener" which means "to have" for example.
If you wanted to wish someone good luck on their job interview that day, you would say:
Deseo que (subjunctive) tengas (dependent verb) buena suerte en la entrevista de trabajo hoy.
The subjunctive verb "desear" which means "to wish/desire" denotes a hypothetical situation. As a result, it affects the dependent verb "tener" by making it switch its usual "-er" ending to an "-ar " ending in the conjugated tú form.