My eraser doesn’t work the way I erase

If you try to erease an image, but you cant, then right click on the image> Break apart. Now your image will be a drawing which you can erease.

You mention a few different scenarios, so maybe it would help to know what some of the expected behavior is with the Eraser tool.

it erases the whole of it

If you double-click on the Eraser tool, it'll delete everything on the Stage at that frame.

or even sometimes it doesn‘t erase

If the object you try to erase is a symbol, nothing will be erased unless you enter symbol-editing mode by double-clicking the symbol on the Stage, going to Edit > Symbols (Ctrl + E), or double-clicking on it from the Library.

it change the shape of it

When you apply the Eraser tool to a shape with a fill (which includes brush strokes made with the Brush tool that paints with a fill), Animate subtracts the shape of the Eraser from the fill shape.

When you apply the Eraser tool to a stroke (which can be the outline of a shape or a stroke drawn with the Line tool, the Pencil tool, or the Paint Brush tool), Animate removes the intersected portion of the path.

So I'm wondering if the issue might be some confusion between the Brush tool and Paint Brush tool, or something else. The only other possibility I can think of would be if you imported a bitmap and wanted to erase lines in that. You would need to break it apart to use the Eraser tool, but Animate would treat the bitmap like a filled shape. So, if that is what you're trying to do, you would need to do the erasing in a raster program like Photoshop.

If these examples don't help, can you post an image of what you're trying to erase and show the Properties panel, so we can see what kind of object it is?

make sure you're erasing a shape, not a symbol.