The word 'phenom' is a slang or informal form for the noun, short for phenomenon (the plural for is phenomena, an irregular plural).
Since the use of the word 'phenom' is relatively modern, the plural form can be a regular plural: phenoms.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "that" is "it."
The singular form is cuff.
it is already in its singular form.
"Stratum" is the singular form of stratum. The plural form is "strata".
The form hypothesis is the singular form. The plural form is hypotheses.
A singular form of dice is a die
The singular form of feet is foot.
The singular form of Oases is, Oasis.
Phenomenon is the singular form of phenomena.
The singular form of chromosomes is a chromatid.
"Wash" is the singular form of "wash."