Чугунную форму поставили в духовку.

Because чугунную форму is the direct object of the verb поставили.

The dictionary forms are:

• чугунная = nominative feminine singular
• форма = nominative singular

But after a transitive verb like поставить when something is being acted on, Russian uses the accusative case for the direct object:

• nominative: чугунная форма
• accusative: чугунную форму

The adjective changes to match the noun in gender, number, and case.

Because this sentence describes movement into the oven, not location inside it.

With в, Russian usually works like this:

• в + accusative = motion into
• в + prepositional = location in

So:

• в духовку = into the oven
• в духовке = in the oven

Compare:

• Чугунную форму поставили в духовку. = They put the cast-iron pan/dish into the oven.
• Чугунная форма была в духовке. = The cast-iron pan/dish was in the oven.

Because духовка is a feminine noun ending in -а, and here it is in the accusative singular after в with motion.

For many feminine nouns of this type:

• nominative: духовка
• accusative: духовку

The same pattern appears with many common nouns:

• книга → книгу
• машина → машину
• комната → комнату

This is a very common Russian pattern. Russian often uses 3rd person plural with no stated subject to mean someone / they in a general or indefinite sense.

So поставили here does not necessarily mean a specific plural they. It often means something like:

• they put
• someone put
• it was put (in natural English translation, though the Russian grammar is active)

This construction is called an indefinite-personal use.

Grammatically, it is active, not passive.

• Чугунную форму поставили в духовку. = active, with an unstated indefinite subject
• Чугунная форма была поставлена в духовку. = passive

English may sometimes translate both in similar ways depending on context, but in Russian they are different structures.

The sentence you have is often more natural and more conversational than the passive version.

Because поставили is perfective, and it presents the action as a completed whole: the pan/dish was put into the oven.

• поставили = completed, one whole action
• ставили = imperfective; could mean repeated action, process, or background action

So:

• Чугунную форму поставили в духовку. = They put the cast-iron pan/dish into the oven.
• Чугунную форму ставили в духовку каждый вечер. = They used to put the cast-iron pan/dish into the oven every evening.

In your sentence, the completed action is what matters, so поставили fits better.

Russian often chooses different verbs depending on the position or the normal way an object is placed.

Very roughly:

• поставить = set/stand something
• положить = lay something down

A форма used for cooking is often treated like a container or dish that you set into the oven, so поставили sounds natural.

If you used положили, it could suggest placing something so that it lies down flat, and in many contexts it would sound less natural for cookware.

English uses put very broadly, but Russian often makes a more specific choice.

Yes. Russian word order is flexible because the case endings already show the grammatical roles.

Your sentence:

• Чугунную форму поставили в духовку.

Other possible orders include:

• Поставили чугунную форму в духовку.
• В духовку поставили чугунную форму.

The basic meaning stays the same, but the focus changes a little.

For example:

• Чугунную форму... puts the pan/dish more in focus as the topic.
• В духовку... highlights the destination.
• Поставили... can sound more event-focused.

Because Russian has no articles.

So чугунную форму can mean:

• a cast-iron pan/dish
• the cast-iron pan/dish

and в духовку can mean:

• into an oven
• into the oven

Context tells you which is meant. This is normal in Russian.

The main word stresses are:

• чугУнную
• фОрму
• постАвили
• духОвку

So one natural pronunciation guide is:

чугУнную фОрму постАвили в духОвку

For a learner, the most important thing is to remember that Russian stress is not fixed, so it has to be learned with each word.