This pillow is more soft than that one.

[Vladv1]

The textbook Advancede Grammar in use by Martin Hewings states that "We can sometimes use "more "as an alternative to the -er form to emphasise the comparison:
"You might think it's dark here but it's more dark in the cellar." And the textbook lists the adjectives clear, cold, deep, fair, rough, soft, true to be used this way.
So based on the aforementioned points, can I say " This pillow is more soft than that one"?

 

[SoothingDave]

Yes. Either way. More soft, or softer.

 

[emsr2d2]

The textbook Advancede "Advanced Grammar In Use" by Martin Hewings states that "We can sometimes use "more "as an alternative to the -er form to emphasise the comparison: "You might think it's dark here but it's more dark in the cellar."

And the textbook lists the adjectives clear, cold, deep, fair, rough, soft, true to be used this way.
So based on the aforementioned points, can I say "This pillow is more soft than that one"?

Note my corrections above. Remember to capitalise book titles correctly and mark them out in some way. I've put the title in quotation marks but you could use italics instead. Don't put a space after opening quotation marks.

 

[Tarheel]

My guess is that 90% of native speakers would say "softer". Also, and I'm not sure why, but I think it would be 95% or higher for "harder".

 

[jutfrank]

There are very few, if any, restrictions on which gradable adjectives you can use 'more' before. It's certainly not limited to 'clear, cold, deep, fair, rough, soft, and true' so it doesn't make much sense to list those particular words. The inclusion of the word 'true' to that list is also odd, as it differs from the others in that it's almost always used as a non-gradable adjective.

 

[jutfrank]

My guess is that 90% of native speakers would say "softer".

The point is that you can say 'more soft', for emphasis, when you feel the word 'softer' does not suffice.

 

[Tarheel]

Yes. Yes, you can.