ふりをする

ふりをする means "pretend": someone acts as if something were true when it isn't. ふり is a noun, so it takes what any noun takes in front of it – a plain verb or an い-adjective directly, a な-adjective with な, a noun with の.

• × 元気ふりをする → 元気なふりをする
• × 学生ふりをする → 学生のふりをする

Which verb form

To fake a state, the verb goes in the た-form or the ている-form: ねた寝たふり is "pretending to be asleep", しんだふり死んだふり is "playing dead". To fake not knowing, hearing or seeing, it goes in the ない-form.

Usage notes

These go beyond the basic "pretend".

ふりをして, then what you do

In the て-form, ふりをして links to what the person does under cover of the pretence.

Dropping を

In casual speech を often drops, leaving ふりする.