Hi sayantan.. :)
When alternate hypothesis is a negation of null then we use Two tailed test.
Suppose, Null hypothesis: true population mean = u_0
Alternate hypothesis : true population mean is
not equal to u_0
And when we have a case like this>
Null hypothesis: true population mean = u_0
Alternate hypothesis : true population mean is
greater than/less than u_0
then, we use one tailed test.
:)