Learn Hardcore Portuguese (Portugal): Um relacionamento forte ajuda quando uma delas passa por uma fase difícil. - A strong relationship helps when one of them goes through a difficult phase.

You would use uma delas when the group you’re referring to is feminine (e.g. only women, or a feminine plural noun like as amigas, as pessoas, etc.).

Why is it delas and not deles here?

Portuguese pronouns agree in gender with the group they refer to:

deles = “of them” (masculine or mixed group)
delas = “of them” (only females, or only feminine nouns)

In this sentence, delas tells us that “they” are feminine: it’s a group of women or some other feminine group already known from the context.

If it were a group of men, or mixed men and women, you would expect um deles / uma deles (depending on the gender of “one”).

If relacionamento is masculine, why do we see uma in uma delas and not um?

Uma delas does not refer back to relacionamento. It refers to one person in a previously mentioned group, typically something like duas amigas, as irmãs, as pessoas, etc.

relacionamento is masculine: um relacionamento forte
uma is feminine because it’s “one” person from a feminine group: uma delas = “one of them (female)”

So:

The article um in um relacionamento agrees with relacionamento (masculine).
The uma in uma delas agrees with the implied feminine noun (e.g. “one (woman) of them”).

They’re agreeing with different nouns, not each other.

What does passar por mean in passa por uma fase difícil?

Here passar por is a set expression meaning “to go through / to experience / to undergo”.

passar alone can mean “to pass” (physically or in time).
passar por
- situation = “to go through (a period / situation)”.

So passa por uma fase difícil = “(she) goes through a difficult phase” / “(she) is going through a difficult time”.

Other common examples:

passar por um problema – to go through a problem
passar por momentos complicados – to go through complicated moments

Why is it passa (present) instead of something like “is going through” in Portuguese?

European Portuguese often uses the simple present for:

General truths or typical situations
- Um relacionamento forte ajuda quando uma delas passa por uma fase difícil.
  → “A strong relationship helps when one of them goes through / is going through a difficult phase (in general).”
Timeless rules or repeated events
Similar to English: “When someone gets sick, we help.”

If you want to emphasise that it’s happening right now, you can use the progressive:

…quando uma delas está a passar por uma fase difícil.
(EP standard progressive: estar a + infinitive)

But with quando, in a general statement like this, the simple present passa is the natural choice.

Could I say está a passar por uma fase difícil instead of passa por uma fase difícil? What’s the difference?

Yes, grammatically you can:

…quando uma delas passa por uma fase difícil.
→ neutral, general, describes a typical situation.
…quando uma delas está a passar por uma fase difícil.
→ focuses more on an ongoing process at a specific time.

In this kind of proverb‑like or general statement (“A strong relationship helps when …”), Portuguese normally prefers the simple present (passa) after quando.

You’d more often use está a passar when talking about a specific current case:

Agora, ela está a passar por uma fase difícil.
“Right now, she’s going through a difficult phase.”

Why do we use por and not another preposition in passar por uma fase difícil?

The preposition por is fixed with this meaning of passar por:

passar por algo = “to go through / undergo something (a period, situation, place, experience)”

Using another preposition would sound wrong or change the meaning. For example:

passar em – often “to pass (an exam)” (passar no exame)
passar a – can mean “to start to” (passar a trabalhar = to start working)

For “go through a difficult phase,” the correct idiomatic choice is passar por uma fase difícil.

Can I say um forte relacionamento instead of um relacionamento forte?

You can say um forte relacionamento, but:

um relacionamento forte is the neutral, most common order.
Moving the adjective before the noun (um forte relacionamento) is more literary or emphatic, and in everyday speech can sound a bit more formal or marked.

So:

um relacionamento forte – standard, natural.
um forte relacionamento – possible, but used for stylistic effect (e.g. in writing, speeches, more formal language).

Meaning-wise, they’re the same: “a strong relationship”.

What’s the difference between relacionamento and relação here?

Both can mean “relationship,” but there are tendencies:

relacionamento
- Often used for ongoing interpersonal relationships, especially emotional or long‑term ones.
- Common in expressions like relacionamento amoroso, relacionamento saudável.
relação
- Broader: any relation / connection (mathematical, logical, social, etc.).
- Still used for personal relationships (ter uma relação com alguém), and in many set phrases.

In your sentence, you could say:

Um relacionamento forte ajuda…
Uma relação forte ajuda…

Both are acceptable, but relacionamento forte feels slightly more like a sustained, personal relationship (e.g. friendship, partnership).

Why is it ajuda (singular) and not ajudam?

The verb agrees with the subject of the sentence:

Subject: Um relacionamento forte (singular)
Verb: ajuda (3rd person singular of ajudar)

So:

Um relacionamento forte ajuda… – “A strong relationship helps…”
If the subject were plural, then you’d use ajudam:
- Relacionamentos fortes ajudam quando… – “Strong relationships help when…”

The form ajudam would be ungrammatical with the singular subject Um relacionamento forte.

Could we start the sentence with Quando uma delas passa por uma fase difícil? Is that still correct?

Yes, that’s perfectly correct and very natural:

Quando uma delas passa por uma fase difícil, um relacionamento forte ajuda.

Portuguese allows either order:

Main clause first (as in your original sentence).
Quando‑clause first, followed by a comma.

The meaning doesn’t change; it’s just a stylistic choice.