| explicitly(adverb, absolutely, certainly, clearly, definitely) |
| fervently(adverb, seriously, vigorously, earnestly, actively) |
| firm(adjective, thoughtful, sober, serious, sincere) |
| fixed(adjective, thoughtful, sober, serious, sincere) |
| fixedly(adverb, absolutely, certainly, clearly, plainly) |
| for a certainty(expression, idiom) |
| for certain(expression, adverb, absolutely, really, certainly) |
| for sure(adverb, definitely) |
| gravely(adverb, down, seriously, vigorously, earnestly) |
| historical |
| honest-to-God(adjective, solid, material, real, genuine) |
| in actuality(expression, adverb, well, absolutely, really) |
| in all seriousness(expression, idiom, adverb, seriously, vigorously) |
| in earnest(expression, adverb, seriously, vigorously, earnestly) |
| in fact(expression, adverb, well, absolutely, really) |
| in point of fact(expression, idiom, adverb, well, absolutely) |
| in reality(expression, adverb, well, absolutely, really) |
| in truth(expression, adverb, absolutely, very, easily) |
| indubitably |
| inexorably(adverb, absolutely, certainly, clearly, perfectly) |