Think → Consider / Deem
think is informal; consider/deem are formal and more precise
Help → Facilitate
help is informal/neutral; facilitate is formal (enable/make easier)
Big → Substantial / Significant
big is informal/vague; substantial/significant are formal and precise
Bad → Detrimental / Adverse
bad is informal/vague; detrimental/adverse are formal
Examples
The committee considered the proposal unsuitable.
Technology can facilitate communication across borders.