"I want to understand Spanish."
Translation:Quiero entender español.
You can't have two conjugated verbs together. Since quiero is conjugated (to yo), you have to use the infinitive form of the next verb, which is entender (to understand). Yo quiero entiendo means 'I want I understand'.
Yes, I too was wondering if there is any difference between 'entender' and 'comprender' . I would prefer using the latter as being cognate with. comprehend.
In Spain they use entender more than comprender. I'm not sure about Latinamerica.
I once read that comprender means to understand what words have been said whereas entender means to understand their meaning, and I've said a good few times "comprendo pero no entiendo".