Can "post" be past tense?

I agree with @Cascabel , "post" is not correctly used as a past tense.

There is the possibility that "post" is used as a subjunctive; and that may refer to something in the past. "The judge ordered that the mayor post the notice in the town square".

I agree with Cascabel and GEdgar: this usage of post seems very unlikely, at least for native speakers.

The Oxford English Dictionary records six different verbs post. They are all listed as regular verbs, however, with no special notes. I also looked at the Merriam-Webster, Lexico, American Heritage Dictionary, Collins, Macmillan, Cambridge, and Wiktionary. They all concur.

As to how you could figure this out on your own online: first, except for the OED, all the other dictionaries are freely accessible, and their not recording any usage of the kind you mention is good evidence that it is not part of Standard English. Second, you can try searching the Corpus of Contemporary American English (COCA) and the British National Corpus (BNC) for POST_v (post as a verb). There will be many hits, but in a couple of hours you could go through them all. Similarly, you could search on Google Books for such things as 'post a', 'post the', 'post it', etc. I doubt you will find any hits generated by native speakers. It is of course hard to prove a negative, but this will be another good indication that the usage you report is not part of Standard English.

The past and past participle of "post": posted

"I post flyers every day" <-- this "post" is present tense and

"every day" implies "habitual actions that are repeated often in your life"

so it means a kind of a habit -- posting flyers -- which you had done in the past, are doing now, and will probably be doing if everything is as usual as the past and as now.

"I post the flyer yesterday" <-- "yesterday" means one day ago, which is "past"

So the verb is not correct and needs to be "posted" or "had posted"

The standard past tense form of post is posted.

There are some verbs ending in [st] where standard English uses a past tense form ending in [st] (it is possible to analyze this as a simplification of [stt], with [t] as an irregular past tense marker instead of the syllabic [əd] that regularly appears with verbs ending in [t] or [d]). For example, cast.

Furthermore, in some accents of English, word-final [t] is commonly dropped after a preceding obstruent consonant, which includes in [st].

This may lead some speakers to use alternative, non-standard past tense forms for some verbs ending in [st].

• One example is text, where it seems that some people use the non-standard past tense form text rather than the standard texted (see With regards to SMS, is the past tense "text" or "texted"?)

• It is also possible to find some examples online of rest used as a past tense form (do a Google search for "sat down and rest") even though the standard past tense is rested.

So it doesn't seem too unlikely to me that you heard someone use post as a past tense form. However, it is not usual and not considered standard.