Learn Italian: Non toccare questo ragno spaventoso!

This is how you give a negative command to one person informally (using tu). While a positive command uses a conjugated verb, telling someone not to do something uses non followed by the full dictionary form of the verb (the infinitive): non toccare.

If I were telling a group of people not to touch the spider, would I still use non toccare?

No, the non + infinitive rule is a special exception just for the singular tu. If you are warning a group of people (voi), you just use the normal conjugation with non in front of it: Non toccate questo ragno!

The word spaventoso means "scary". Is the -oso ending related to English "-ous"?

Yes! The suffix -oso is very common in Italian for turning a noun into an adjective, similar to "-ous" or "-y" in English. Here, it comes from the noun spavento (fright or panic). Because ragno is a masculine singular noun, the adjective takes the matching -o ending.

Why doesn't questo drop the final "o" and use an apostrophe here?