Learn Russian: Этот механизм представляет собой основу системы. - This mechanism represents the foundation of the system.

Even though this phrase acts like the verb 'to be', the verb представлять is transitive. It directly acts on основу, putting it in the Accusative case. The phrase literally translates to 'presents the foundation by means of itself'. This is a major difference from formal verbs like являться, which take the Instrumental case.

Does the pronoun собой change if the subject is feminine or plural?

No, it stays exactly the same. Собой is the Instrumental case of the reflexive pronoun себя (itself/oneself). Because it always refers back to the subject regardless of gender or number, it remains собой. Only the verb changes form, for example, if the subject were plural (представляют собой).

How does представляет собой differ in nuance from является?

Both are used in formal, academic, or technical contexts. However, представляет собой specifically emphasizes the intrinsic nature, composition, or internal structure of the subject. It is frequently used when defining what something constitutes or forms in and of itself.

Can I separate the words представляет and собой in this sentence?

It is highly recommended not to. The verb and pronoun in представлять собой are a set phrase and are very tightly linked. While Russian word order is famously flexible, splitting these two words or reversing their order sounds unnatural. Treat them as a single, inseparable unit.