Is this blazer good for business wear?

Yes. The navy plaid and single-breasted cut are formal enough for most offices, and the pattern reads as intentional rather than casual. It is less conservative than a solid navy jacket, so it suits business casual and creative workplaces best.

How does the modern fit compare with a slim or classic fit?

Modern fit sits between the two: closer through the chest and waist than a classic cut, with more room than a slim fit. If you are between sizes or broad-shouldered, try one size up.

What can I wear with a navy plaid sport coat?

Grey trousers, chinos in stone or navy, dark denim and white or light blue shirts all work. Keep the tie simple, since the plaid already carries the outfit.

Is it a suit jacket or a separate blazer?

It is a sport coat meant to be worn as a separate, not half of a matched suit. Matching it to trousers of the same plaid is not the intent.

The blazer itself. No trousers, shirt or tie are included.

Can I wear it in spring and summer?

It suits spring dinners and warm-weather events, but the fabric weight is not specified. If you run hot, check the material and lining before committing to outdoor summer wear.