Añadimos el aditivo mientras el tambor está vacío.

Grammatically, añadimos can mean either we add (present) or we added (preterite), because those two forms are identical for many -ir verbs in the nosotros form.

Here, the context makes it clear that it means we add:

• it sounds like a process description or instruction
• the rest of the sentence describes a current state: mientras el tambor está vacío

So in this sentence, añadimos is understood as the present tense.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• añadimos = we add
• so nosotros añadimos is usually unnecessary unless you want emphasis or contrast

For example:

• Añadimos el aditivo... = normal
• Nosotros añadimos el aditivo... = more emphatic, like we are the ones who add it

Yes, other verbs are possible, but añadir is a very natural choice, especially in Spain and in technical or formal contexts.

Some differences:

• añadir = to add; very standard and common in instructions
• agregar = also to add, but often feels a bit more common in Latin American Spanish than in Spain
• echar = to put in / pour in / throw in; more general and sometimes less precise

In an industrial or procedural sentence, añadir sounds especially appropriate.

Spanish often uses the definite article where English might use the, an, or even no article at all, depending on context.

Here, el aditivo suggests:

• a specific additive already known in the process, or
• the additive that is normally used in this procedure

So el aditivo sounds natural if both speaker and listener know which additive is meant.

If you said un aditivo, it would sound more like:

• any additive
• one additive among several possible ones

Mientras means while.

It introduces an action or condition happening at the same time as the main action:

• Añadimos el aditivo = the main action
• mientras el tambor está vacío = the simultaneous condition/state

So the idea is that the additive is added during the period when the drum is empty.

Because estar is used for a state or condition, while ser is used more for identity, classification, or inherent characteristics.

Here, vacío describes the drum’s condition at that moment:

• el tambor está vacío = the drum is empty right now / at this stage

Using es vacío would sound unnatural here, because being empty is not the drum’s essential nature; it is just its current state.

Because the sentence is describing a real, factual situation.

With mientras, Spanish normally uses:

• indicative for something real or actual
• subjunctive in more limited cases, often with a different meaning such as as long as

Here, the speaker is stating a real condition:

• mientras el tambor está vacío = while the drum is empty

So está in the indicative is the natural choice.

Yes. Tambor literally means drum, but in technical or industrial contexts it can refer to:

• a drum-shaped container
• a barrel-like unit
• a rotating drum in a machine

So even if the English translation says drum, the intended meaning may be an industrial container or machine part rather than a musical drum.

Yes. Spanish is fairly flexible with word order, especially with clauses like this.

The given sentence is completely natural:

• Añadimos el aditivo mientras el tambor está vacío.

But you could also say:

• Mientras el tambor está vacío, añadimos el aditivo.

Both are correct. The difference is mostly one of emphasis:

• starting with Añadimos... focuses first on the action
• starting with Mientras... focuses first on the condition/time frame

They have accent marks for different reasons.

• está has an accent because the stress falls on the last syllable: es-TÁ
  • It also helps distinguish it from esta, which can be a determiner meaning this
• vacío has an accent to show that the í is stressed and pronounced separately:
  • va-CÍ-o
  • without the accent, Spanish pronunciation rules would tend to make it sound different

So the accents are important both for pronunciation and for correct spelling.

In Spain, a typical pronunciation would be approximately:

• añadimos → ah-nya-DEE-mos
• tambor → tam-BOR
• está → es-TA
• vacío → va-THEE-o in most of Spain, because c before i is pronounced like th
  • in Latin America, it would be more like va-SEE-o

A few useful pronunciation points:

• ñ sounds like the ny in canyon
• á shows the stressed syllable
• vacío has three syllables: va-cí-o, not two