Mi madre siempre es muy discreta cuando habla de ese asunto.

Because ser is normally used for a person’s general characteristic or usual way of being, while estar is more often used for temporary states or conditions.

In Mi madre siempre es muy discreta..., the sentence describes your mother as a consistently discreet person, especially when this topic comes up. That is why es sounds natural.

• es discreta = she is a discreet person / that’s part of her character
• está discreta = unusual, and generally not what you would say here

Also, siempre strongly suggests a habitual quality, which fits ser very well.

Here discreta means discreet, tactful, or careful about what she says, especially regarding a sensitive topic.

It does not usually mean discrete in the mathematical or technical English sense.

So in this sentence, muy discreta suggests that the mother speaks in a reserved, careful, tactful way when the subject comes up.

Because the adjective must agree with the noun it describes.

• madre is a feminine singular noun
• so the adjective must also be feminine singular: discreta

Compare:

• Mi madre es discreta
• Mi padre es discreto

This is standard adjective agreement in Spanish.

Spanish often omits subject pronouns when the verb ending already makes the subject clear.

In cuando habla de ese asunto, the verb habla can mean he speaks, she speaks, or you speak formally, but in context it clearly refers to mi madre.

So Spanish prefers the more natural, less repetitive version:

• Mi madre siempre es muy discreta cuando habla de ese asunto.

You could say cuando ella habla de ese asunto, but it would usually sound unnecessary unless you wanted extra emphasis or contrast.

The subject is still mi madre.

The structure is:

• Mi madre siempre es muy discreta = main clause
• cuando habla de ese asunto = subordinate clause

Even though mi madre is not repeated, it is understood as the subject of habla.

So the meaning is:

• My mother is always very discreet when she talks about that matter.

Because this sentence talks about a habitual, repeated action, not a future or hypothetical one.

• cuando habla = when she talks / whenever she talks
• cuando hable = when she talks in the future, or in certain dependent contexts

Here, siempre shows a repeated pattern:

• She is always very discreet whenever she talks about that issue.

So the present indicative (habla) is the correct choice.

Compare:

• Cuando habla de ese asunto, es muy discreta. = whenever she talks about it, she is very discreet
• Cuando hable de ese asunto, será muy discreta. = when she talks about it in the future, she will be very discreet

Siempre usually goes before the verb it modifies, especially in simple sentences.

So:

• Mi madre siempre es muy discreta... = natural
• Mi madre es siempre muy discreta... = possible, but less common and a bit more marked or literary

Placing siempre before es is the most neutral and natural option.

The demonstrative ese usually points to something not right here with the speaker, often something already known, mentioned before, or mentally a bit distanced.

So:

• este asunto = this matter, closer to the speaker
• ese asunto = that matter, a bit more distant
• aquel asunto = that matter over there / more distant still

In this sentence, ese asunto sounds natural because it refers to that particular issue, probably one both speaker and listener already know about.

Also, in real use, ese asunto can create a slight sense of distance, which fits a delicate or sensitive topic well.

Yes, tema could also work, but there is a slight nuance.

• asunto = matter, issue, affair
• tema = topic, subject

Asunto often sounds a little more serious, delicate, or specific than tema. In a sentence about being discreet, ese asunto fits very well because it suggests something possibly sensitive.

Compare:

• hablar de ese asunto = talk about that matter / issue
• hablar de ese tema = talk about that topic

Both are correct, but asunto may sound a bit more serious or delicate.

Both can be correct, but hablar de is usually the most common and natural way to say to talk about.

• hablar de algo = the standard, everyday choice
• hablar sobre algo = also possible, sometimes a bit more formal or more like speak on/about

So:

• habla de ese asunto = very natural
• habla sobre ese asunto = correct, but slightly less everyday in many contexts

For ordinary conversation, hablar de is usually the safest choice.

Normally, no, not in this context.

With discreto/discreta, Spanish usually uses ser to describe a person’s character or habitual way of behaving:

• Mi madre es muy discreta

Using estar with discreta would sound unusual and would not normally express the intended meaning. Native speakers would strongly prefer ser here.

Yes, in effect it can.

Literally, siempre means always, but in this kind of sentence it combines with cuando to give the idea of a repeated pattern:

• Mi madre siempre es muy discreta cuando habla de ese asunto.

This can be understood as:

• My mother is always very discreet when she talks about that matter.
• or more naturally in English:
• My mother is very discreet whenever she talks about that matter.

So siempre + present often expresses habitual behavior.

Yes. Spanish allows some flexibility in word order.

For example:

• Mi madre siempre es muy discreta cuando habla de ese asunto.
• Cuando habla de ese asunto, mi madre siempre es muy discreta.

Both are correct. The second version gives a little more focus to cuando habla de ese asunto.

The original order is very natural and neutral.

Modern standard Spanish uses the possessive directly before the noun:

• mi madre
• tu hermano
• su casa

Using an article before the possessive in this position is not standard modern Spanish.

So:

• mi madre = correct
• la mi madre = not standard in modern general Spanish

Spanish does sometimes use structures like la madre mía, but that is different, more emphatic, and much less neutral than the normal mi madre.

It belongs with habla.

The structure is:

• hablar de algo = to talk about something

So:

• cuando habla de ese asunto = when she talks about that matter

It is not describing discreta directly. Instead, it tells you what she talks about when she is being discreet.