It's not always the same. One example: When he returns I will talk to him. Quand il reviendra je lui parlerai. Cuando regrese le hablaré. French uses future indicative (or sometimes present indicative) in the "when" clause, Spanish uses subjunctive. For another example, the imperfect subjunctive has basically disappeared from spoken French, with the present subjunctive being used even for past situations. So you will not be able to use your Spanish "instincts" mechanically by any means. Of course, both languages use the subjunctive to express doubts, wishes, or counterfactual situations, but then that is more or less the definition of "subjunctive." The devil is always in the details. Spanish will definitely help you, especially with vocabulary, and even some of the more conservative verb forms are pretty similar if you ignore the different spelling and focus only on the sounds (e.g. "serai" and "seré" [I will be]). But you have to be careful not to rely too much on your stronger language when learning a new, similar one.