I here people say "/dev/sda1 is mounted on /" .Shouldn't the wording me "/dev/sda1" is mounted as /" ?

Another question regarding the same subject - the directory /dev/ to exist, the parent / has to exist first. It is like chicken and egg situation. Can someone explain or refer to the article that explains this please. I am new to these Linux concepts.

See your OED.

(Frankly, this is a mite of a semantic sophistry that would best be posed to the Grammer Girl.)

Since this is your first post a belated welcome to LinuxQuestions.

In a nutshell it is all in the start up process. In most distributions the root filesystem is first created by the initrd and once the kernel is up running the init process mounts the root partition via the /etc/fstab. I am far from being a kernel expert so excuse any errors...

https://en.wikipedia.org/wiki/Linux_startup_process

When you ring your mum, do you say you are talking to your mum - or do you say you are talking to a microphone chip ?. Better to get comfortable with accepted usage rather than trying to change the world.

As for / versus /dev/, the kernel creates the root (i.e. the kernel is running before the root directory is created) and the standard directories. Then it goes looking for what you want to mount where.
In all likelihood, the more answers you get the more confused you will be.

I prefer "/dev/sda1 is mounted at /".

Never depend on a spoken language when you're looking for precision.

It's a directory in its own right - note that it is chmod 755, so users have access to it.

The / is mounted "by magic" first, normally read-only.
Later it is remounted as stated in the fstab.

I don't know if all Linux distributions do it the way Debian does it, but the basic sequence of events is:

1) The bootloader loads up the Linux kernel and an initial ramdisk image. This has a "/" ramdisk that fits entirely within ram.

The ramdisk image is pretty minimal, but it has enough file system stuff to mount the real / (either over nfs or local drive) and read /etc/fstab from it

2) The linux kernel starts executing boot up scripts in the initrd. It mounts the real / to something like /rootmount

3) After doing this, it does some other stuff and then chroots to the real / at /rootmount. Before doing this, it'll scoot other mounts to below /rootmount or something like that

So fundamentally, the "chicken and egg" problem is solved by first loading a "chicken" entirely within ram in a minimalistic ramdisk install. Then it mounts the "real" stuff and scoots over into the "real" / and mount points.