Learn Spanish (Latin America): Cuidando bien el cactus, mantienes la raíz sana. - Caring for the cactus well, you keep the root healthy.

In Spanish, gerunds (words ending in -ando or -iendo) are frequently used to set up a condition, similar to an "if" clause. Saying Cuidando bien el cactus... is a very natural and concise way of saying "If you take good care of the cactus..." (Si cuidas bien el cactus...) or "By taking good care of the cactus...".

Since cactus ends in an 's', is it singular or plural here?

It is singular here, which you can tell from the singular article el. Cactus is an invariable noun, meaning the word itself doesn't change when pluralized. To talk about multiple cactuses (or cacti), you would simply change the article to plural: los cactus.

Why is there a written accent on the 'i' in raíz?

The accent prevents the strong vowel (a) and the weak vowel (i) from blending into a single syllable (a diphthong). It forces the stress onto the i, splitting the word into two distinct syllables: ra-ÍZ. Note that when pluralized, the z changes to a c, but the accent remains for the exact same reason: raíces.

Why use sana instead of saludable to mean "healthy"?