Does 'Comparative' always follow 'than' ?

Hi, I was always curious about this grammar.

A native English speaker said you always have to put 'than' when you use 'comparative'.

For example,
'This is more expansive than this.'

However, I think there are some cases where I can use comparative sentences without using 'than'.

For example,
'This is good, but I think it will be more expensive.'
'I want to try this, but it will be more difficult.'
'There is another bag, but I think this is still better.'

What do you think?

Thank you for reading this.

Question

Does 'Comparative' always follow 'than' ?

Hi, I was always curious about this grammar.

A native English speaker said you always have to put 'than' when you use 'comparative'.

For example, 
'This is more expansive than this.'

However, I think there are some cases where I can use comparative sentences without using 'than'.

For example,
'This is good, but I think it will be more expensive.'
'I want to try this, but it will be more difficult.'
'There is another bag, but I think this is still better.'

What do you think?

Thank you for reading this.

Answer

Hi Bi Hana,
In the first two of the last three sentences above, the comparison can be understood only in context, meaning we (readers) can only understand what is being compared if we know what came before. So, if those sentences are part of a longer paragraph, or if they follow some previous information, then they are perfectly understandable as written. The last of the final three sentences above contains the thing being compared (a bag) first in the sentence. It could be rewritten as, "I think this (bag) is better than the other bag."
If there is no context in a sentence at all, then you would need the word "than" to follow your introductory word in the comparison, whatever that word may be: more, better, stronger, funnier, smarter, etc.