The phrase "very difficult to determine" is grammatically correct and can be used in written English.
Example: The true origin of the ancient artifact is very difficult to determine.
The value, Barley added, "is very difficult to determine.
And it's very difficult to determine what the play is ultimately about.
"It's very difficult to determine who is really in control now," said a Western diplomat here.
Becoming very difficult to determine where the dirt stops and I begin.
As with practically everything else about him, the truth is very difficult to determine.
However, it is still very difficult to determine non-invasively the normal skin impedance.
Exactly what losses or injuries are "natural and proximate" results is often very difficult to determine.
The latter is very difficult to determine in situ, resulting in increased uncertainty in the models.
Each couple's risk depends on the individuals' particular genetic makeup, she said, which means "it's very difficult to determine".
"It's very difficult to determine, in some cases, on this battlefield who is a combatant and who is a civilian.
However, it is very difficult to determine the hybridization and structure of amorphous carbon experimentally during sliding.
