The verbs hisi (feel physically or emotionally) and ganjon (feel emotionally) are transitive verbs that are turned into copular verbs by adding the suffix -cu.
Transitive verb
Kam yu le hisi geotreme?
Did you feel the earthquake?
Copular verb
Mi hisicu bimar.
I feel sick.
Recently, it occurred to me that much like ambitransitive verbs, -cu could be optional for these verbs, thereby working much like the English verb feel. I had previously considered this possibility, but decided against it since the optional use of -cu wouldn't work in the case of the copular verbs for the five senses: okocu, orecu, nasacu, xetocu, pifucu. The reason -cu must be obligatory for these verbs is that these are the type of transitive verbs that often take a null direct object.
Transitive verb with null object
Mi oko bon.
I see well.
vs
Copular verb
Mi okocu bon.
I look good.
However, although hisi and ganjon are also transitive verbs, they are not the null-object type. To illustrate, the verb feel in an English sentence like I feel well/good would always be interpreted a copular verb, never as a transitive verb. This demonstrates that whereas -cu must be obligatory for the verbs denoting the five sense, it is safe to optionally apply -cu for the copular meaning of hisi and ganjon.
Another reason that the optional use of -cu for hisi and ganjon is convenient is to avoid the awkward -cugi in hisicugi and ganjoncugi, meaning to cause to feel or make (somebody) feel. Instead, we can just say hisigi and ganjongi.
Yu ganjon(cu)gi mi hox.
You make me feel happy.
I suspect that even without officially introducing this usage here, hisigi and ganjongi would've eventually prevail as intuitive derivations.
