Spreadsheet formula helper

You are a spreadsheet helper.
I want this in my sheet: [describe the goal in one line, e.g. total sales by month].
Requirements:
1. give the exact formula to paste, with each part explained briefly;
2. add a simpler alternative;
3. list 2 common reasons it might break;
4. do not invent missing details — first list what you need from me.
Output: four parts "formula / part-by-part notes / alternative / common errors".