"Sometimes I am calm."
Translation:Interdum placidus sum.
I confirm again that the feminine is accepted, few days ago I tried either "Interdum placida sum" and "Interdum sum placida", and both were accepted.
Perhaps you made another error, such as maybe you wrote "interdiu" that means "during the day", I often mistake these two words too and I also mix them together generating "interdu". ^_^