These can all be fine. Which one you use depends on context: It was morning when I heard the sound of thunder. (This emphasizes the time of day.) It was a morning in April when I heard the sound of thunder. (The speaker may not remember which morning, only that it was in April.) It was Sunday morning when I heard the sound of thunder. (This emphasizes the day of the week.) It was a Sunday morning in April when I heard the sound of thunder. (The speaker remembers it was a Sunday in April, but not which Sunday, ie, first? second? third? fourth?). On Sunday morning we're going out for brunch. (This implies you plan to go to a restaurant to eat brunch the very next Sunday.) We're going to meet for brunch on a Sunday morning next month. (You plan to meet to eat brunch on a Sunday morning the following month, but you have not yet determined WHICH Sunday, ie, first? second? third? fourth?).

Great question! "It was morning," "It was Sunday morning," and "On Sunday morning" are commonly used in both spoken and written English. "It was a morning" is typically used to tell stories, especially in books, to create a scene or set a mood. For example: "He called me on Sunday morning." → More informal, commonly used in everyday English. "It was *a* cold Sunday morning, and the world outside was covered in a blanket of snow." → More descriptive and artistic, often used to tell a story. I hope this helps!

Here’s a quick breakdown: 1. "It was morning" sounds natural. You don’t need the article "a" in this case because "morning" is used in a general sense. 2. "It was Sunday morning" sounds more natural. You don’t need the article "a" because you're referring to a specific time of the week. 3. "On Sunday morning" sounds natural. You don’t need "a" because you're talking about a specific morning in the context of a particular day. So the most natural phrases are: * "It was morning." * "It was Sunday morning." * "On Sunday morning."