[*] 168. The tenses of the Present System are made from the Present Stem as follows:—1 [*] a. In the Present Indicative the personal endings are added directly to the present stem. Thus,—present stem arā-: arā-s , arā-mus, arā-tis. [*] b. In the Imperfect Indicative the suffix -bam, -bās, etc. (originally a complete verb) is added to the present stem: as, arā-bam, arā-bās, arā-bāmus.
[*] Note.--The form †bam was apparently an aorist of the Indo-European root BHU (cf. fuī , futūrus , φύω, English be, been), and meant I was. This was added to a complete word, originally a case of a verbal noun, as in I was a-seeing; hence vidē-bam . The form probably began in the Second or Third Conjugation and was extended to the others. The a was at first long, but was shortened in certain forms (§ 167).
[*] c. In the Future Indicative of the First and Second Conjugations a similar suffix, -bō, -bis, etc., is added to the present stem: as, arā-bō, arā-bis, monē-bō .[*] Note.--The form †bō was probably a present tense of the root BHU, with a future meaning, and was affixed to a noun-form as described in b. N.
[*] d. In the Future Indicative of the Third and Fourth Conjugations the terminations -am, -ēs, etc. (as, teg-am, teg-ēs, audi-am, audi-ēs) are really subjunctive endings used in a future sense (see e). The vowel was originally long throughout. For shortening, see § 167. [*] e. In the Present Subjunctive the personal endings were added to a form of the present stem ending in ē- or ā-, which was shortened in certain forms (§ 167). Thus, ame-m , amē-s , tegā-mus , tega-nt .[*] Note 1.--The vowel ē (seen in the First Conjugation: as, am-ë-s ) is an inherited subjunctive mood-sign. It appears to be the thematic vowel e (§ 174. 1) lengthened. The ā of the other conjugations (mone-ā-s, reg-ā-s, audi-ā-s) is of uncertain origin.
[*] Note 2.--In a few irregular verbs a Present Subjunctive in -im, -īs, etc. occurs: as, sim , sīs , sīmus , velim , velīs , etc. This is an old optative, ī being a form of the IndoEuropean optative mood-sign yē- (cf. siem, siēs, siet, § 170. b. N.). The vowel has been shortened in the first and third persons singular and the third person plural.
[*] f. In the Imperfect Subjunctive the suffix -rem, -rēs, etc. is added to the present stem: as, amā-rem , amā-rēs , monē-rem , tege-rem , audī-rem .[*] Note.--The stem element -rē- is of uncertain origin and is not found outside of Italic. The r is doubtless the aorist sign s (cf. es-se-m, es-sē-s) changed to r between two vowels (§ 15. 4). The ē is probably the subjunctive mood-sign (see e).