Learn Spanish (Latin America): El profesor está de pie cerca de la puerta.

In Spanish, physical postures like standing, sitting, or kneeling are usually expressed with specific phrases rather than a continuous "-ing" verb. To say someone is "standing," you use the phrase estar de pie (literally "to be of foot").

Why do we need the word de in cerca de? Can I just say cerca la puerta?

You need the de! By itself, cerca just means "close" or "nearby" (for example, él está cerca means "he is nearby"). When you want to specify what something is close to, you must use the complex preposition cerca de.

Why do we use está instead of es in this sentence?

There are two reasons to use the verb estar here. First, it tells us where someone is located (cerca de la puerta). Second, it describes a temporary physical state or posture (de pie). Both location and temporary states always use estar rather than ser.

If the teacher was near a masculine object, like "the desk" (el escritorio), how would the sentence change?