Verbs in -μι differ in inflection from verbs in -ω in the present and second aorist systems and (rarely) in the second perfect system. Verbs in -μι have the following peculiarities of inflection:
a. The endings -μι and -σι (for original -τι) occur in the present indicative active: τίθη-μι, τίθη-σι; φη-μί φη-σί.
b. The 3 plural present indicative active has generally the ending -α_σι, from α-αντι, as τιθέα_σι, ἱστᾶσι. So in the 2 perf. active ἑστᾶσι.
c. The 3 plural of active past tenses has -σαν: ἐτίθε-σαν.
d. The imperative ending -θι is sometimes retained: φα-θί, στῆ-θι; some forms never had it: τίθει, ἵστη.
e. The middle endings -σαι and -σο regularly retain ς: τίθε-σαι, ἐτίθε-σο.
N.—But not in the subjunctive or optative; and usually not in the second aorist; as τιθῇ for τιθέη-σαι, τιθεῖο for τιθέ-ι_-σο, ἔθου for ἔθε-σο.
f. The infinitive active has -ναι: τιθέ-ναι, διδό-ναι; the 2 aorist active has -εναι rarely: θεῖναι for θέ-εναι, δοῦναι for δό-εναι.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].