Learn Hardcore Spanish (Spain): Ahora la bombilla nueva funciona bien y dejamos la linterna junto al interruptor por si acaso. - Now the new light bulb works well and we leave the flashlight next to the switch just in case.

la bombilla nueva: more neutral; it simply means the bulb that is new (as opposed to the old one). This is the most natural option here.
la nueva bombilla: tends to add a bit of emphasis, more like the new bulb (as opposed to the previous one), sometimes with a slightly more subjective or contrastive feel.

In everyday speech, la bombilla nueva is what you’d normally say in this context.

Why is funciona used here instead of está funcionando?

Funciona (simple present) describes a general state or regular situation: it works / it functions (properly now).
Está funcionando (progressive) focuses on the action right now, at this very moment.

With machines or devices, Spanish usually uses the simple present to say they work properly in general:

La bombilla funciona bien. = The bulb works well (now / in general).
La bombilla está funcionando would sound like you’re talking about a specific ongoing operation (e.g., We’re testing it and it is currently working), which isn’t needed here.

Why do we say funciona bien and not just funciona?

You could say Ahora la bombilla nueva funciona, and it would be understandable, but it sounds a bit incomplete, as if you were contrasting it with not functioning at all.

By adding bien, you clearly mean it works well / properly, not just it turns on. It’s the most natural way to say it works correctly.

Could I use trabaja instead of funciona for the bulb?

No. For machines or devices, Spanish almost always uses funcionar:

La bombilla / el ordenador / el coche funciona bien.

Trabajar is for people and sometimes for complex systems (a company works, a system works), but not for a light bulb. La bombilla trabaja bien sounds wrong.

Why is dejamos in the present tense? In English I might say we left the flashlight…

Dejamos in the present here is describing a present habit or current arrangement: we (normally) leave / keep the flashlight by the switch.

If you wanted a completed past action, you’d say, for example:

Y hemos dejado la linterna junto al interruptor. = And we have left the flashlight next to the switch.

So in this sentence, dejamos is more like we keep / we leave it there (as a rule).

Could I say ponemos la linterna instead of dejamos la linterna?

You could, but the nuance changes:

ponemos la linterna junto al interruptor = we put/place the flashlight next to the switch (focus on the act of placing it there).
dejamos la linterna junto al interruptor = we leave/keep the flashlight next to the switch (focus on leaving it there and not moving it).

In this context, dejamos fits better because the idea is to keep it there just in case.

In Spain, does la linterna mean lantern or flashlight?

In Spain, la linterna normally means flashlight (what Americans call a flashlight, British often call a torch).

If you say linterna in Spain, people will picture a hand‑held battery light, not an old‑fashioned hanging lantern.

What does junto al interruptor mean exactly? How is junto a different from al lado de?

junto al interruptor = next to / right by the switch.

junto a and al lado de are very similar and usually interchangeable. Both mean next to / beside.
junto a can sound a bit more formal or precise; al lado de is very everyday and common.

So you could also say: dejamos la linterna al lado del interruptor.

Why is it al interruptor and not a el interruptor?

Al is simply the contraction of a + el:

junto a + el interruptor → junto al interruptor

Spanish always contracts a el → al and de el → del, unless el is a pronoun, which it isn’t here. So a el interruptor is not correct in this context.

What does the phrase por si acaso mean, and why por, not para?

Por si acaso is an idiomatic expression meaning just in case or in case something happens. Literally it’s like saying for if (by chance).

You must use por, never para, here. Para si acaso is not idiomatic Spanish.

It’s used when you do something as a precaution:

Lleva un paraguas, por si acaso. = Take an umbrella, just in case.

Is there a difference between por si and por si acaso?

Yes, but it’s small:

por si: in case
por si acaso: just in case (adds a bit more of the “you never know” feeling)

In many everyday contexts, you can switch them with no real change:

Dejamos la linterna junto al interruptor por si acaso.
Dejamos la linterna junto al interruptor por si se va la luz. (in case the power goes out)

Why is ahora used here? Could I say ya instead?

Both ahora and ya are possible, but they focus on slightly different things:

Ahora la bombilla nueva funciona bien: emphasizes now (at this point), contrasting with before.
Ya la bombilla nueva funciona bien: closer to the bulb works now / at last it works, often after some waiting or effort.

In many contexts they overlap, but ahora sounds very natural here to mark the change in the present situation.

Why do we need the definite articles la bombilla, la linterna? Could we omit them?

You can’t omit them here. In Spanish, concrete countable nouns almost always need an article when they’re specific:

la bombilla nueva = the new bulb (a specific one)
la linterna = the flashlight you both know about

Saying bombilla nueva funciona bien or dejamos linterna is ungrammatical in this context.

Why are bombilla and linterna feminine? Is there a rule I can use?

In Spanish, grammatical gender is mostly arbitrary and must be learned with each noun:

la bombilla → feminine → la bombilla nueva
la linterna → feminine → la linterna

There are some patterns (many nouns ending in -a are feminine), but there are lots of exceptions. The safest habit is always to learn each new noun together with its article: la bombilla, el interruptor, etc.