Elegantiae terminorum

Flagrans burning is from a fire, blazing with breath or flame. But fragrans fragrant signifies a scent.
Fenero I lend to you, that is, I give money to you at interest.
Feneror I borrow from you, that is, I receive money from you at interest.
We say fero and profero iras I carry and show anger, but not affero.
Festus-a-um pertains to a festival: as dies festus holiday. But festivus-a-um pertains to other things: as festiva oratio pleasant and witty speech. Festivum caput a witty person.
Flere to weep is with tears. Bemere to groan is to grieve in the chest.
Plorare to wail is to disclose one's grief with a mournful and outpouring voice. Plangere to beat/lament is to afflict oneself through the striking of limbs or grief. Eiulare to wail is to weep with a raised voice, which is more typical of women. Vociferari to shout is to show not only grief but indignation with an unbridled voice. Lamentari to lament is to testify to one's own ruin with a querulous and mournful voice. Luctus mourning is grief that is shown externally by the very face and appearance; sometimes it is taken for mourning attire.
Frequens frequented is a place where many dwell: as a town; or where many frequent: as a forum or square. It is also said of a living thing: as "the soldier is frequent," that is, crowded and abundant. "The people are frequent," "the students are frequent," that is, numerous and many.
Fretus sum I am confident is not taken for positus sum I am placed.
Fio I become has compounds: as calefio, frigefio, which are not in use among the learned; but in their place calefio, frigefio are used. Many, however, are in use: as madefio, patefio. Those that end in -sco signify a passion caused internally. But those that end in -fio signify a passion inflicted externally; as calefco from within, calefio from without.
Fiendus-a-um that which is to be made is not used; but faciendus-a-um.
Fidem habeo tibi I have faith in you, that is, I believe your words. Fideiussor guarantor. But do fidem I give pledge is to promise to do something. Perfidus perfidious is one who has violated a given pledge. Infidus, however, is one who is not to be trusted. Supra vel ultra fide beyond belief is more than can be believed.
Fore, fores, foret is the same as essem, esses, esset. But fore is not only to be, but to be about to be. And thus it cannot be joined to a participle of future time; so it should not be said "Scio Petrum fore lecturum" I know Peter will be about to read, because it is nonsense. If one adds the participle in -dus, this passes into the force of a noun; thus it is nonsense.
Gratiarum actio thanksgiving is proper and more elegant. And it is joined with habeo.
Gratias ago I give thanks is good; gratias habeo I have thanks, refero gratias I return thanks, grates ago I offer thanks. Avoid saying gratias tibi reddo I return thanks to you, because it is barbaric.
Grates is poetic, rarely oratorical. Redire in gratiam to return to favor is to be reconciled with the inanimate.