El suplemento es por el atril.

Because both nouns are masculine singular, so they take el:

• el suplemento
• el atril

In this sentence, both are also specific: we are talking about a particular supplement and a particular lectern/music stand, not just any one.

A good habit is to learn new nouns together with their article:

• el suplemento
• el atril

That helps you remember gender naturally.

Here, por is being used with a location idea, meaning something like around, by, or in the area of.

That is important because por often does not mean just one thing. In Spanish it can express:

• movement through an area
• cause/reason
• exchange
• approximate location

In this sentence, it suggests a general area, not a very precise position. So it feels looser than expressions such as:

• junto al atril = right next to the lectern
• al lado del atril = beside the lectern

So por el atril sounds more like somewhere around the lectern area.

This is a very natural question, because English speakers quickly learn that location is usually expressed with estar in Spanish.

If you are talking about the physical location of an object, está would usually sound more natural:

• El suplemento está por el atril.
• El suplemento está junto al atril.

So if the idea is simple physical position, many speakers in Spain would prefer está.

Using es here can sound unusual without more context. In other words, if your first reaction was Shouldn’t this be estar?, that is a sensible reaction.

It is understandable, but it is not the most neutral everyday wording if you are simply describing where something is.

In Spain, a speaker would often be more likely to say:

• El suplemento está por el atril.
• El suplemento está al lado del atril.
• El suplemento está junto al atril.

So the sentence you have is possible to analyse, but for normal physical location it may sound a bit odd or context-dependent.

Only if the meaning changes.

This is a classic learner difficulty:

• por often relates to cause, movement, area, means, exchange, or approximate location
• para often relates to purpose, destination, deadline, or intended recipient

So if the intended idea is location near the lectern, para would not work.

A native English speaker may see for in English and want to choose para, but Spanish does not map onto English that neatly. With por and para, you always need to think about the function, not just the English translation.

Normally, no.

In this sentence, atril is a normal countable noun referring to a specific object, so Spanish wants the article:

• por el atril

Leaving out the article would sound incomplete or ungrammatical in standard usage.

Spanish uses articles more often than English in many situations, so this is something English speakers need to get used to.

Not necessarily.

In Spain, atril can refer to:

• a lectern
• a music stand
• a book stand
• any stand used to hold reading material at an angle

The exact meaning depends on context. So when learners meet atril, they should think of a broader idea: a stand for holding something to read or look at.

Yes, but the basic order here is the most straightforward:

• El suplemento = subject
• es = verb
• por el atril = prepositional phrase

Spanish word order is flexible, so you can move parts around for emphasis, but not every version sounds equally natural. For example, if you were talking about location, a speaker might say:

• Por el atril está el suplemento.

That puts emphasis on the place first. But again, with a location meaning, está would usually sound better than es.

Atril is stressed on the last syllable:

• a-TRIL

And suplemento is stressed on:

• su-ple-MEN-to

So the full sentence is pronounced roughly with the main stresses on:

• El su-ple-MEN-to es por el a-TRIL

For a native English speaker, the key point is that atril ends in -l, so by the normal stress rules it is stressed on the last syllable.