When I press the space bar it deletes the letters after it [duplicate]
This is caused by the Insert key on your keyboard, it replaces the letters to the right as you explained.
Simply press the Insert key and it will deactivate the replace mode. Pressing it the second time reactivates it.
I guess you are in replace mode, just press the "Insert" key on your keyboard.
From comments (Thanks @FabioTurati)
The cursor is different: in Insert mode it's a vertical bar placed between two letters, whereas in Overtype mode it's an underscore, and it is under one letter (the one which will be overwritten if you type something)