29.210 The Aorist Active System of τίθημι

Make sure you look at Croy section 210, where he provides the aorist forms for τίθημι. As Croy explains, the aorist past tense indicative is formed regularly from the third principal part, which is unusual because of the kappa in the stem instead of the sigma that you would expect:

aorist past active ἔθηκα, ἔθηκας, ἔθηκε, ἐθήκαμεν, ἐθήκατε, ἔθηκαν

aorist past middle ἔθηκάμην, ἔθήκω, ἐθήκατο, ἐθηκάμεθα, ἐθήκασθε, ἐθήκαντο

What is odd about this verb is that all the other forms of the aorist system (aorist infinitives, participles, subjunctive and imperative) use a simple second aorist stem without the kappa. In effect, the stem consists of the letter θ- .

For example, the aorist active participle is θείς, θεῖσα, θέν, and the aorist active infinitive is θεῖναι. Croy provides some other examples that you should note in this section.

You can tell immediately that you are dealing with aorist forms because they are built on the simple θ- stem, rather than the reduplicated τιθ- stem of the present system (and since the perfect system is built on a different reduplicated stem, τεθ- , you can quickly recognize the perfect forms as well).