He started cleaning voluntarily.

A new approach is needed to solve this perpetual problem.

His house has a very modern design.

His secondary role greatly contributed to the success of the team.

He always keeps in mind the phrase 'If one learns without thinking, it is a waste.'

Thinking sensibly, that plan will not succeed.

This is a preliminary plan.

He is interested in diachronic linguistics studies.

He is interested in synchronic linguistics.

That theory is unscientific and has no evidence at all.