Learn Hardcore Portuguese (Portugal): Se mexeres o café sem cuidado, ele pode salpicar a mesa. - If you stir the coffee carelessly, it can splash onto the table.

Se mexeres o café… = If you stir the coffee (at some future moment)…
Using Se mexes o café… (present indicative) sounds wrong/nonnative in this conditional/future context.

So the pattern is:

English: If you stir the coffee, it can splash. (present)
Portuguese: Se mexeres o café, ele pode salpicar. (future subjunctive)

What tense/mood is “mexeres” exactly, and how is it formed?

Mexeres is 2nd person singular (tu) – future subjunctive of mexer.

For regular verbs, the future subjunctive is usually formed from the 3rd person plural preterite (eles mexeram):

Take eles mexeram
Remove -am → mexer-
Add the endings: –, -es, –, -mos, -em

So for mexer:

(eu) mexer
(tu) mexeres
(ele/ela/você) mexer
(nós) mexermos
(eles/elas/vocês) mexerem

You mainly see this tense after conjunctions like se, quando, enquanto, logo que, assim que when they refer to the future:

Quando mexeres o café, prova-o. – When you stir the coffee, taste it.
Se mexeres o café depressa, vai salpicar. – If you stir the coffee fast, it will splash.

Could I say “Se mexer o café…” instead of “Se mexeres o café…”?

You can, but it changes who the subject is and/or makes it ambiguous.

Se mexeres o café… → clearly “if you (tu) stir the coffee…”
Se mexer o café… (future subjunctive) could mean:
- Se eu mexer o café… – if I stir the coffee
- Se ele/você mexer o café… – if he/you (você) stir the coffee

Because Portuguese often drops subject pronouns, se mexer o café without a pronoun is ambiguous.
In European Portuguese, if you mean “tu”, se mexeres is the natural, clear choice.

What’s the function of “pode” in “ele pode salpicar a mesa”? Could I use “vai salpicar” instead?

Pode is the verb poder = “can / may”, and it expresses possibility, not certainty:

ele pode salpicar a mesa = it *might/can splash the table*

Using vai (from ir) would be more like “will / is going to”, which sounds more certain:

ele vai salpicar a mesa = it *will splash the table* (very likely / expected)

So:

pode salpicar → possible, but not guaranteed
vai salpicar / salpicará → more certain, more predictive

What does “sem cuidado” literally mean, and are there other natural ways to say this?

Literally, sem cuidado = “without care”.

In this sentence:

Se mexeres o café sem cuidado…
≈ If you stir the coffee *without being careful / carelessly*.

Common alternatives (European Portuguese):

Se mexeres o café com cuidado, … – If you stir the coffee carefully, …
Se mexeres o café de forma descuidada, … – in a careless way
Se mexeres o café à pressa, … – if you stir the coffee in a hurry (implies less care)

But sem cuidado is simple, natural, and very common.

Why is “ele” used for “o café”? I thought “ele” meant “he”.

In Portuguese, ele/ela refer to any noun (not just people) and agree with its grammatical gender:

o café (masculine) → ele
a mesa (feminine) → ela

So ele here is essentially “it (masculine)”:

Se mexeres o café sem cuidado, ele pode salpicar a mesa.
→ If you stir the coffee carelessly, *it can splash the table.*

Using ele for an inanimate object is completely normal and very common.

Could I omit “ele” and just say “Se mexeres o café sem cuidado, pode salpicar a mesa”?

Yes, that’s also possible and natural in Portuguese.

Se mexeres o café sem cuidado, ele pode salpicar a mesa.
Se mexeres o café sem cuidado, pode salpicar a mesa.

Both are fine. Portuguese often drops subject pronouns when the subject is clear from context.
Using ele can:

make the subject slightly more explicit,
sometimes add a tiny bit of emphasis (that coffee can splash the table).

But in everyday speech many people would simply say “pode salpicar a mesa”.

Why is it “salpicar a mesa” and not “salpicar na mesa”?

Because salpicar here is used as a transitive verb: the table is the direct object, the thing that gets splashed.

salpicar a mesa = to splash/speckle the table (to get drops on the table)

Using na mesa makes mesa a location (on the table), not a direct object:

salpicar na mesa would sound more like to splash on/at the table as a place, and is unusual in this context.

So:

O café pode salpicar a mesa. – The coffee can (end up) speckling/staining the table.
If you wanted to emphasize the location, you would more naturally say something like:
O café pode cair na mesa. – The coffee can fall on the table.

Is “mexer o café” the standard way to say “stir the coffee” in European Portuguese? Doesn’t “mexer” just mean “to move”?

In European Portuguese, mexer is very commonly used to mean “to stir” food or drink:

mexer o café – stir the coffee
mexer a sopa – stir the soup
mexer o molho – stir the sauce

More generally mexer does mean “to move”, “to handle”, “to fiddle with”, depending on the preposition:

mexer em alguma coisa – to mess with / fiddle with something
- Não mexas em nada! – Don’t touch anything!

But with food/drinks as a direct object, mexer naturally means “stir” in Portugal.

Could I replace “salpicar” with other verbs like “espirrar” or “respingar”?

You can, but the nuance changes slightly.

salpicar – to splash/speckle with small drops; very natural here.
respingar – to splatter/splash in small drops; also possible:
- …pode respingar na mesa / pode respingar a mesa.
espirrar – can mean “to splash/spurt”, but its main meaning is “to sneeze”, so it can sound odd or humorous in this context.

In European Portuguese, for this sentence:

salpicar or respingar are your best choices.
espirrar would usually be avoided here unless you really want that double meaning.

Is there any difference between European Portuguese and Brazilian Portuguese in a sentence like this?

Main points:

Pronouns and verb forms
- Portugal (tu):
  Se mexeres o café sem cuidado, ele pode salpicar a mesa.
- Brazil (você):
  Se você mexer o café sem cuidado, ele pode respingar na mesa.
In Brazil, você is common and takes 3rd person verb forms (mexer, not mexeres).
Vocabulary / structure
Both salpicar and respingar exist in Brazil, but respingar na mesa is very common.
The basic structure and meaning stay the same, though.

Since you’re focusing on Portuguese from Portugal, Se mexeres o café… with tu is the most typical pattern.

Why do we say “a mesa” (the table) and not just “mesa”?

Portuguese uses the definite article (o, a, os, as) more often than English does.

Here, a mesa refers to a specific, known table (the one in front of you), so:

ele pode salpicar a mesa = it can splash *the table*

Saying just “pode salpicar mesa” (without the article) sounds ungrammatical in this concrete, specific sense.

The article would usually only be dropped in some fixed expressions or very generic, abstract uses (which is not the case here).

Could “café” here mean “coffee shop” instead of the drink?

Not in this sentence.

mexer o café clearly refers to stirring a drink.
You don’t “stir” a café (coffee shop), but you do stir coffee in a cup.

So here o café = the coffee (the drink), masculine noun, hence ele later on.