Please tell me the singular form of this word.

In English, adding 's' after a noun makes it a plural form.

After using the computer for a long time, my shoulders became stiff.

He explained in detail about the structure of the sphenoid bone.

This butterfly valve is used to control the flow of fluid.

The characteristic of annelids is that their bodies are divided into ring-shaped segments.

She is learning the grammar of the present progressive form.

He drew a straight line using a T-square.

This shape is a parallelogram.

This shape is a parallelogram.