Phalaenopsis orchids (the type pictured) appreciate "bright indirect" light, which while sounding a big vague, essentially means not dim light but not too many hours of direct sunlight either. Indoors, a semi-sunny window should be fine for them, or sitting in front of a sunnier window if it has a sheer curtain in front of it. The plant's current location, if it normally grows off to the side of that window, looks too dim for good long-term growth and reblooming.

The yellowing leaf pictured looks like the oldest leaf, and periodic yellowing and shedding of old leaves is normal for all plants, including orchids. (Since orchids grow relatively slowly compared to other houseplants, this leaf shed is just slower to occur.) Sometimes old leaves shed earlier than they usually would if a plant is stressed due to over-watering, under-watering, under-fertilizing, or insufficient light.

Phalaenopsis grow best when their potting mix is allowed to become mostly dry between waterings, since their roots have some water-storage capability. They do not want to stay constantly damp, though when watering is needed, it's best to flush the pot with plenty of water so the excess freely drains out of the bottom of the pot. This ensures all roots have been re-moistened and it also flushes out any "salts" (mineral buildup from tap water or fertilizer residue) so they don't reach levels high enough to harm roots.

The type of potting mix for orchids like this needs to be free-draining, so they should not be planted in typical potting soil used for other houseplants. Common materials used for potting them include long-fiber sphagnum moss (what your plant is in) and/or orchid bark (also called fir bark). Either material can be mixed with some perlite and horticultural charcoal to help keep it airy, since orchid roots need lots of oxygen around their roots. In the wild, their roots would glue themselves to a tree trunk out in the open (not covered in any type of soil), soaking up moisture as rainwater drips down the trunk, carrying trace amounts of nutrients with it. In our homes, the humidity is lower overall, so roots dry out faster, hence why we put them in pots (for ease of growing them too).

You could move it into a pot with more holes if you prefer, and if you are inclined to water often, that will help avoid over-watering (to some extent, anyway). Bark-based potting mixes allow the plant to dry faster, and sphagnum moss-based potting mixes retain moisture for longer, so you can use either depending on how you tend to water. If you think you tend to over-water, try a bark type of orchid mix in a pot with more drain holes (orchid pots have holes or slits on their sides in addition to the bottom, whether ceramic, terra cotta, or plastic). Unglazed terra cotta wicks water from the potting mix, which can be a benefit for pots that stay wet, because it will help the roots dry out faster. If you think you tend to under-water, keep using the moss in a pot with drain holes only on the bottom. Customize it how you want, though, and just adjust your watering practices to compensate. The mix can get fairly dry about an inch down into the pot (or more) before the plant will need a good drenching. Make sure any water pooling in the crown (the center, where leaves attach to the main stem) is dabbed-up to dry off after watering so it doesn't sit wet and encourage rot.

Orchids are not heavy nutrient users ("less is more" tends to work well with them), but if the plant hasn't been fertilized in several months, consider using a liquid orchid fertilizer formulation and follow dosage/dilution information (if you buy a concentrate) on the package.

Miri

Thanks so much, Miri! this is very helpful.

You're welcome. Enjoy your weekend.