"My parents decided that we would celebrate my birthday."
Could anyone explain to me why this is "would" and what grammar shall i learn to make sure?

The use of "would" in the sentence "My parents decided that we would celebrate my birthday" is an example of the future in the past. In English, when we talk about the past and discuss decisions, intentions, or plans that were made in the past, we often use "would" to indicate the future action at that time.

Here's a breakdown of the sentence:

- My parents decided (in the past): This is the main action in the past. The decision was made at some point before the birthday.

- that we would celebrate my birthday: This is the future action that was decided upon in the past. "Would" is used here to indicate the future from the perspective of the past. It's a way of expressing what was planned or intended to happen.

To better understand this construction, it's helpful to learn about reported speech, indirect speech, and the use of modal verbs (like "would") for expressing future in the past. Additionally, studying the past tenses and their usage in English will provide a solid foundation for comprehending these structures.