What could it be: つもりでいた？?

Question

I need some help with the sentence below. Things like ～ている、～でいる when finishing the verb as in [似]{に}ている、[読]{よ}んでいる are okay for me. But next sentence is something else, because the ending でいた is standing after a noun, not a verb.

結婚した後、日本語を勉強するつもりでいたが、生活のために始めた仕事は、フランス語を教えること。

• Could you please explain, why でいた was used here, in this sentence?
• And when we can usually use grammar like this?

Answer 1

This is simply a valid combination of two things you already know:

• te-form + いた forms the past-progressive
• the te-form of the copula is で

so we have:

• 日本語を勉強するつもりだ -> intend to study Japanese
• 日本語を勉強するつもりでいた -> was intending to study Japanese