Have you ever encountered this kind of problem? That it is impossible to type the numbers 7, 8, 9 as well as the letters U, I, O, J, K, L, M and maybe even other keys
It's likely to have something to do with an older macOS feature that mimics an old IBM-style keyboard feature..
The keypad lock key and its history
Keyboards have vestiges of their origins on typewriters and mechanical calculators. But we are quite ignorant of all this. The "Shift" key comes originally from the typewriter and allows you to make a transition between upper and lower case letters. And you surely know very well the function of the key "Caps Lock" also called "Caps Lock", which when it is activated, and that you must type a password, your Mac displays a message of alert and of reminder.
The numeric keypad lock key, also commonly referred to as "Num Lock," was a product of the Middle Ages, when mechanics and numerics met. Some IBM computer keyboards did not have separate arrow keys, but did have a numeric keypad for quick number entry. IBM then doubled down by adding the "Num Lock" key which allows you to switch the numeric keypad into direction keys to facilitate the movement of the cursor on the screen. It was long before the days of the mouse..
On some computers, including older Macs, the numeric keypad lock key has a different function. As a numeric keypad was missing, “Num Lock” transformed part of the main keyboard by adding a pseudo-numeric keypad. (Sometimes there was a single "Num Lock" key along with "Clear" and "Shift" keys, it was a key to invoke, much like the F6 key on Mac.
The pseudo-keyboard layout was based on sets of four keys starting with 7, 8, 9, 0 and going up to the M key. With the mode on, you can still type 7, 8, 9, 0 and bring up these characters as they are in the same places. On the other hand, it is different with the keys U, I, O, P, J, K, L, semicolon, M, command, period, slash, which support keyboard functions..
Since 2008, Apple has stopped using the numeric keypad lock key via F6. It is therefore impossible for you to activate this key accidentally, as it no longer exists. On the other hand, it is still available in some computers in Boot Camp, but Apple has not specified which ones.
On the flip side, some developers created a numeric keypad lock app - simply called NumLock - that could enable this old feature on Mac OS X; but it looks like it hasn't been updated in years, and I haven't tested it on Mojave yet.
Mouse keys and their modern use
Apple always gives you the option of going back to the IBM days through an accessibility feature which is, on macOS, called "the mouse keys." When the mouse keys are turned on from the "Accessibility" preferences of the "Mouse & Trackpad" section, the number keys on the keyboard change into diagonal, left, right, up, and down cursor movements in small increments, as you click. long press a key the cursor speed increases. To click with the “mouse”, press the 5 key; to click and hold, press the 0 key; to release, press the period key on the numeric keypad.
However, if you're using a laptop computer, the mouse keys sort of hijack a subset of that old IBM structure. That is, keys 7, 8, 9, I, O, P, J, K, L, M, and dot have the same layout as a number keyboard (789, 456, 123, 0.).
A reader on Macworld discovered that even when he plugged an external numeric keypad into the keyboard, the mouse keys were still activated. But you can turn them off through the Accessibility Preferences window. If you haven't changed the default settings, then you can just press the "Option" key five times to scroll. A message will then appear on the screen to let you know that the IBM function is disabled. If you want to reactivate it, proceed in the same way.
If you have any other questions on the subject, or if you know of other tips of the genre, feel free to leave a comment in the space below.