D, in general, is pronounced as in English, except that the tongue should touch the teeth rather than the palate.
[Pompei. Comm. ad Donat. Keil. v. VI. p. 32.] D autem et t quibus, ut ita dixerim, vocis vicinitas quaedam est, linguae sublatione ac positione distinguuntur. Nam cum summos atque imos conjunctim dentes suprema sui parte pulsaverit d litteram exprimit. Quotiens autem sublimata partem, qua superis dentibus est origo, contigerit t sonare vocis explicabit.
But when certain words in common use ending in d were followed by words beginning with a consonant, the sound of the d was sharpened to t; and indeed the word was often, especially by the earlier writers, written with t, as, for instance, set, haut, aput:
[Mar. Vict. I. iii. 50.] D tamen litteram conservat si sequens verbum incipiat a vocali; ut haud aliter muros; et haud equidem. At cum verbum a consonante incipit, d perdit, ut haut dudum, et haut multum, et haut placitura refert, et inducit t.
F is pronounced as in English except that it should be brought out more forcibly, with more breath.
[Keil. v. VI. p. 31.] F litteram imum labium superis imprimentibus dentibus, reflexa ad palati fastigium lingua, leni spiramine proferemus.
Marius Victorinus says that f was used in Latin words as ph in foreign.
Diomedes (of the fourth century) says the same:
[Diom. Keil. v. I. p. 422.] Id hoc scire debemus quod f littera tum scribitur cum Latina dictio scribitur, ut felix. Nam si peregrina fuerit, p et h scribimus, ut Phoebus, Phaethon.
And Priscian makes a similar statement: