Serial Available confuse..

Using Arduino Programming Questions
1

what is the difference between
if (Serial.available())
and
if (Serial.available() > 0)
and
if (Serial.available() > 18)
and
if (Serial.available() <10)
and
if (Serial.available() == 3)
???

2
3

if (Serial.available())
and
if (Serial.available() > 0)

No practical difference at all.
Serial.available does not return negative values, so any non-zero value means data is available to read.

I'll leave you to figure out the rest.

4

I have read that link but it is not complete to explain every possibility between 0,18,< or > or == ...

I have read a book that code like this:

if (Serial.available() > 18) {
char led = Serial.read();
if (led == 'S'){
temp1 = Serial.read();
temp2 = Serial.read();
temp3 = Serial.read();
temp4 = Serial.read();
temp5 = Serial.read();
temp6 = Serial.read();
temp7 = Serial.read();
temp8 = Serial.read();
temp9 = Serial.read();
}
}

what its purpose to put 18 there?

5

None what so ever. The code waits untilat least 19 bytes are in the buffer and then reads ten of them.
However in the contex of the whole program it might make sense.

6

I have read that link but it is not complete to explain every possibility

Given that the serial buffer can contain up to 64 characters, all possibilities would take too much page space to enumerate, so they probably listed a few, and left the reader to figure out the rest.

what its purpose to put 18 there?

To ensure there are at least 19 characters to read before going about reading them.