A co 'oni nachazi kazde pondeli na zaprazi jidlo. Dobra vila jim ho tam nechava'. To je ten opakujici se dej, ktery momentalne neprobiha. Kdyby ten dej totiz momentalne probihal, protoze treba nachazeji houby, nebo se hrabou na skladce a nachazeji jidlo (oni jsou treba psi), tak by tam s nejvetsi pravdepodobnosti byl anglicky prubehovy cas, ktery jasne naznacuje, ze se neco deje zrovna ted "they are finding food". Oni se zrovna ted prohrabavaji skladkou a nachazeji jidlo. A ktery jste mozna jeste neprobiral.
We'd ideally need a Czech teacher to explain this really properly, but I don't think there is one.
The grammar is called Slovesný vid and it's the way how we deal with the continuity of activities. In English there is present perfect/simple/continuous and and in Czech there are actually two verbs for one activity, one is "sloveso dokonavé" and one is "sloveso nedokonavé".
The main thing is: Dokonavá slovesa express activities, that are restricted and express only past or future. Here it's "najít": "Našel jsem" = "I (have) found" and "Najdu" = "I will find".
You can't express presence with that, because "Dokonavé sloveso vyjadřuje takový děj, který bude ukončen, protože už se v něm nemůže pokračovat dále, protože bylo dosaženo cíle, kvůli kterému vůbec děj začal." = "Dokonavé sloveso expresses the action, that will be finished, because it can't continue any more - the (desired) goal has been fulfilled"
Well and Nedokonavá slovesa express activites, that don't have any end and express all three tenses - presence, past and future. Here it's "nacházet": "Nacházel jsem" = "I was finding", "Nacházím" = "I am finding" and "Budu nacházet" - "I will be finding".
I guess that's enough for now. If you want to know which is which, you can look here: http://prirucka.ujc.cas.cz/en/?slovo=nach%C3%A1zet&Hledej=Search - when you click on the blue "SSČ" or "SSJČ", then the green "dok" and "ned" sais if it is "dokonavé" or "nedokonavé"
And if you feel like reading Czech, than here is the article that I needed to read myself in order to be able to explain it: http://www.mojecestina.cz/article/2013110101-slovesny-vid-dokonava-a-nedokonava-slovesa
I agree with the comment above. The two verbs are similar, but not identical. The difference between them is the grammatical aspect.
However, I can imagine a situation where "najdou" would be correct as well. For example when you're telling a story to someone and you're only using present simple. Then as the individual actions follow - one after another, you'd say "they come to the shore and find a boat" = "oni přijdou k pobřeží a najdou loďku".
The important is that "dokonavá slovesa" (najít - oni našli, oni najdou) are used for "one time" finished actions (they find it and they have it, happens once and the action is done). Also since it happens only once they can either find it in the past or in the future (there is no present tense).
On the other hand, "nedokonavá slovesa" (nacházet - oni nacházeli, oni nachází, oni budou nacházet) are used for "repeated" unfinished actions (they find it once, twice, three times, etc). This process could have started in the past, can be present and can continue in the future. All 3 forms (past, present, future) are possible.
I am not a Czech teacher by any mean but am afraid that even Dokonava verbs can be used in present tense. One that comes to mind is 'to go'.
Ja jdu - i go, more like I am going because it definitely is happening right now and it happens once. Vs. Ja chodim - i go, which can be used for stuff like 'i go to school'. every day, i go. In past tense Ja jsem sel (past of Ja jdu) was one time thing. Ja jsem chodil was something that, again, was happening over and over.
But not all such verbs exist in present tense. Welcome to Czech!
Ale find je 'najit'. Akorat v cestine je pritomny cas slova 'najit' slovo 'nachazet', protoze 'najit' proste pritomny cas nema. Nas hlavni preklad je 'Oni nachazeji jidlo'. V minulem case by to bylo 'Oni nasli jidlo".
jestli si to nepletete se slovem Hledat, coz se ceskym mluvcim casto stava.