Learn Hardcore French: J’ai fait venir une technicienne, car le client ne se débrouille pas. - I had a technician come, because the customer is not managing.

It’s the causative: the subject causes someone else to do the action. J’ai fait venir [quelqu’un] = “I had [someone] come” / “I called [someone] in.” It implies you arranged for the person to come, not that you came yourself.

Why is it j’ai fait venir and not something with être, since venir usually takes être?

Because the conjugated verb is faire, not venir. In the causative, you conjugate faire (which always takes avoir in compound tenses) and keep the second verb in the infinitive: j’ai fait venir. By contrast, je suis venu means “I came.”

Should fait agree with technicienne (e.g., faite)?

No. With the causative faire + infinitif, the past participle fait is invariable: it never takes feminine or plural endings. Examples: Je l’ai fait venir. Je les ai fait venir.

Why is it une technicienne here?

Because the technician is female: masculine un technicien, feminine une technicienne (note the double n + e). The indefinite article introduces a new person; la technicienne would refer to a specific one already known.

Could I say J’ai appelé une technicienne or J’ai fait appel à une technicienne instead? What’s the nuance?

J’ai appelé une technicienne: you phoned/contacted her; coming over is implied only by context.
J’ai fait venir une technicienne: you arranged for her to come to you.
J’ai fait appel à une technicienne: “I called on/engaged a technician,” a bit more formal; focuses on requesting services.

What’s the difference between car, parce que, and puisque?

car = “for,” more formal/written; often after a comma; states a reason.
parce que = neutral “because,” most common in speech.
puisque = “since/as,” used when the reason is assumed to be known/obvious.
You could also write: ... parce que le client ne se débrouille pas with essentially the same meaning.

How does se débrouiller work? Is it different from débrouiller?

Yes. Se débrouiller means “to manage, cope, get by.” Without the reflexive pronoun, débrouiller means “to untangle/sort out (something).”
Examples: Il se débrouille en français (he can get by in French). Je débrouille les fils (I untangle the wires).

Does ne se débrouille pas mean “not at all” or just “not well”?

Default is “isn’t managing.” To say “not well,” use ne se débrouille pas bien or pas très bien. For “not at all,” use ne se débrouille pas du tout. For “no longer,” use ne se débrouille plus.

Where do the negation and reflexive pronoun go?

Order: ne + reflexive pronoun + verb + pas → le client ne se débrouille pas. In casual speech, ne is often dropped: Le client se débrouille pas, but keep ne in formal writing.

How do I replace une technicienne with a pronoun?

Specific person: Je l’ai fait venir (“I had her come”). Plural: Je les ai fait venir.
“One (of them)” idea: J’en ai fait venir une (“I had one come”).
In all cases, fait stays invariable.

Can I use other tenses with this structure?

Yes:

Present: Je fais venir une technicienne (I’m having one come now).
Imparfait: Je faisais venir une technicienne (I used to/was having one come).
Futur: Je ferai venir une technicienne (I’ll have one come).
Passé récent: Je viens de faire venir une technicienne (I just had one come).

Should it be le client or la cliente?

Use le client for a specific male client, la cliente for a specific female client. The masculine can serve as a generic in some contexts, but if the gender is known, match it.

Why isn’t there a preposition before venir (e.g., faire à venir)?

After faire in the causative, the next verb is a bare infinitive with no preposition: faire venir, faire réparer, faire entrer, etc.