Learn Farsi: آنها به چدن کروم اضافه می‌کنند تا بیشتر مقاوم شود. - They add chromium to cast iron so that it becomes more resistant.

In Farsi, را is used to mark a specific or definite direct object. Here, they are adding chromium as a general substance, not a specific batch of "the" chromium, so the definite marker is left out.

What is the word order doing here? Why is به چدن (to cast iron) placed before the direct object کروم?

Farsi word order is quite flexible. While Subject + Direct Object + Indirect Object + Verb is standard, placing the indirect object prepositional phrase (به چدن) before a general, indefinite direct object (کروم) is very common and flows naturally. Saying آنها کروم به چدن اضافه می‌کنند is also completely correct.

The sentence uses بیشتر مقاوم (bishtar moghavem). Could I use the suffix -تر (-tar) instead?

Yes, absolutely! مقاوم‌تر (moghavem-tar) is the standard way to say "more resistant" and is completely natural here. Using بیشتر (more) as a separate word before the adjective is simply an alternative comparative form that speakers sometimes use for emphasis or with longer adjectives.

Why is the final verb شود (shavad) in the subjunctive mood rather than the indicative?

The conjunction تا (ta) translates to "so that" or "in order to" in this context. Because it introduces an intention, purpose, or desired outcome rather than an established fact, it requires the verb that follows it to take the subjunctive mood.