učvrstiti

1. to strengthen; to consolidate
   Ova pobeda je učvrstila njihovo mesto na vrhu tabele. — This win consolidated their place at the top of the table.
   Zajedničko putovanje je učvrstilo naše prijateljstvo. — The trip together strengthened our friendship.
2. (of an object) to fasten; to secure
   Moramo da učvrstimo policu pre nego što stavimo knjige. — We need to secure the shelf before we put the books on it.

The imperfective is učvršćivati.