i want to add some alphabets at starting in a cell and copy them down
Adding a custom list is probably the way to go. Try this:
1. Go to File > Options > Advanced. Scroll down until you see Edit Custom Lists... in the General section and click to open.
2. Under List entries, click into the field and type A, then press Enter, type B, and repeat the process. Click Add and OK when done.
3. Go back to your spreadsheet and type A into cell A1, and B into cell B1.
4. Highlight both cells and left click onto the bottom right of the cell to Autofill. Drag down until happy.
Do this second
5. You should see the letters come out correctly.