At first, I seemed more or less unable to say oui instead of si, but luckily I seem to have overcome this.
But I still forget to include the subject pronoun sometimes, and as both German and English never drop them, I concluded I must be thinking in Spanish when this happens.
The other thing is the placement of pronouns with modal verbs. I have a hard time to remember to put them between modal verb and infinitive (as in "Je dois me brosser les cheveux"), because this is the position that would not work in Spanish.
Has anyone experienced similar problems?