The differences in the French endings -at, -et, -act, and -ect for words that have English equivalents (such as contract, contact, suspect, subject, and object) come down to their etymological history and phonetic evolution.

1. Latin Origins and French Evolution

Most of these words come from Latin, where their root forms had endings like -actus or -ectus (for nouns) and -are (for verbs). However, French phonetic changes over time influenced how they ended up being spelled and pronounced.

A. Words Ending in -act / -ect (Unchanged from Latin)

Some words kept their original Latin endings, particularly those derived from:

• Latin -actus (e.g., contractus -> contrat in French, contract in English)
• Latin -ectus (e.g., objectus -> objet in French, object in English)

These forms were typically simplified in Old French to -at or -et rather than keeping the Latin -ct.

However, when these words were later borrowed into English (often through academic Latin in the Renaissance), English restored the -ct ending, leading to the divergence you see today.

Examples in French:

• contrat (vs. English contract)
• contact (same in English)
• objet (vs. English object)
• suspect (same in English)
• sujet (vs. English subject)

B. Why -at and -et Instead of -act and -ect?

In French, there was a tendency to drop final -us from Latin (contractus → contrat), and in some cases, -ct softened into -t or -et. This phonetic simplification was part of a broader trend in Old French.

• Words like contrat and objet come directly from Latin but underwent these simplifications.
• Words like contact and suspect were re-borrowed later from Latin without alteration, which is why they keep -ct.

2. Borrowing vs. Retention in English and French

• French often simplified words over time (e.g., contrat instead of contract).
• English, borrowing many of these words later (sometimes through Middle French or directly from Latin), often kept the Latin -ct ending.

3. Summary

• Words that evolved naturally in French from Latin often dropped -ct in favor of -t or -et (e.g., objet, contrat).
• Words borrowed into French at a later stage, or those that retained a learned Latin influence, kept -ct (e.g., contact, suspect).
• English often preserved or reintroduced the Latin spelling (contract, object).