How to see it
A sentence holds one complete idea. It starts with a capital and ends with a mark: a full stop for a statement, a question mark for a question, an exclamation mark for strong feeling. Names, places, days, months and titles always take capitals. Seasons do not. One idea, one sentence.
A worked example
Listen for where each idea ends. On Saturday we went to York: full stop. Saturday is a day and York is a place, so both take capitals. It rained all day: full stop. Did we mind? That asks, so a question mark. We sang all the way home: full stop. Four sentences, four capitals.
The common mistake: joining two sentences with only a comma
The trap is joining two sentences with only a comma. We went to York, it rained all day. Both halves stand alone, so a comma is not enough. Use a full stop and a capital, or a joining word like and.
Try one
You ↑A remembered the ↑B tickets, didn't you ↑C I left them ↑D on the table.
Where should a question mark be added?
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Right: C
That makes the first part a question. Then a new sentence starts, with I, which always has a capital.