Present participle; Past participle - strengthening - strengthened Present - strengthen - strengthen - strengthens - strengthen - strengthen - strengthen Past - strengthened - strengthened - strengthened - strengthened - strengthened - strengthened Future - will strengthen - will strengthen - will strengthen - will strengthen - will strengthen - will strengthen Present conditional - would strengthen - would strengthen - would strengthen - would strengthen - would strengthen - would strengthen Present Perfect - have strengthened - have strengthened - has strengthened - have strengthened - have strengthened - have strengthened Past Perfect - had strengthened - had strengthened - had strengthened - had strengthened - had strengthened - had strengthened Future Perfect - will have strengthened - will have strengthened - will have strengthened - will have strengthened - will have strengthened - will have strengthened Past conditional - would have strengthened - would have strengthened - would have strengthened - would have strengthened - would have strengthened - would have strengthened Imperative - - - strengthen - - - - - - - -