Learn Hardcore Spanish (Spain): El termómetro está en el baño; a menudo lo uso en invierno. - The thermometer is in the bathroom; I often use it in winter.

Use estar for the location of people and things: El termómetro está en el baño. Use ser for events: La fiesta es en el parque. With objects, es en is wrong for location.

Why say en el baño and not en baño?

Spanish normally uses an article with places. En el baño = “in the bathroom” (a specific one). If you truly mean “in a bathroom (any),” say en un baño. Bare en baño sounds incorrect.

Does baño mean “bathroom” or “bath”? What if I mean “bathtub”?

El baño commonly means “the bathroom” (the room).
“Bath” as in taking a bath: tomar un baño.
“Bathtub” is la bañera.
In Spain you’ll also hear el cuarto de baño (full bathroom) and el aseo (toilet/half-bath).

What does lo refer to here, and why not la or le?

Lo is the direct object pronoun for masculine singular things. It refers to el termómetro (masculine).

Feminine thing: la (e.g., la crema → la uso).
Le is indirect object or (in Spain) used for masculine people (leísmo), but not for things like a thermometer.

Do I need to say the subject pronoun? Should it be Yo lo uso?

No. Spanish drops subject pronouns unless you want emphasis or contrast. Lo uso is the default. Yo lo uso emphasizes “I (as opposed to others) use it.”

Can I omit the pronoun and say Uso el termómetro en invierno?

Yes. If you repeat the noun, you don’t need the pronoun. The pronoun avoids repetition when the noun is already clear:

With noun: Uso el termómetro en invierno.
Without repeating the noun: Lo uso en invierno.
You can also topicalize: El termómetro, lo uso en invierno.

Where else can the pronoun go?

Before a conjugated verb: Lo uso.
Attached to an infinitive/gerund/affirmative command: Voy a usarlo / Lo voy a usar, Estoy usándolo / Lo estoy usando, Úsalo; negative commands: No lo uses.

What does a menudo mean? Are there alternatives?

It means “often.” Common alternatives: con frecuencia, frecuentemente (a bit formal), normalmente (often = “usually”). Another natural option is Suelo usarlo en invierno (“I tend to use it…”), using soler.

Where can I put a menudo?

Beginning: A menudo lo uso en invierno.
End: Lo uso en invierno a menudo.
Middle (after the verb phrase): Lo uso a menudo en invierno.
Don’t split the clitic and verb: Lo a menudo uso sounds wrong.

Is the semicolon (;) natural in Spanish? Could I use a period or a comma?

The semicolon is fine and formal. A period is very common: El termómetro está en el baño. A menudo lo uso en invierno. A plain comma between two independent clauses is usually incorrect; use y or a semicolon instead.

Why isn’t invierno capitalized? Should there be an article?

Seasons in Spanish aren’t capitalized unless they start a sentence. En invierno is the usual phrasing. En el invierno is also correct but less frequent; it may sound a bit more specific or contrastive.

Why do está and termómetro have accents, and what about baño?

Está needs an accent to mark stress on the last syllable (and to distinguish it from esta “this [fem]”).
Termómetro has an accent to keep stress on mó: ter-MÓ-me-tro (otherwise it would default to the penultimate syllable).
Ñ in baño is a distinct letter; the tilde isn’t an accent but part of the letter.

Could I say Hay un termómetro en el baño instead?

Yes, but it changes the meaning. Hay un termómetro… introduces/explains existence (“there is a thermometer…”). El termómetro está… assumes a specific thermometer is already known in the context.

Is El termómetro se encuentra en el baño okay?

Yes. Se encuentra / se halla sound a bit more formal or written, meaning “is located.” Está is the most neutral.

Why en in en invierno? Can I use durante or por?

En commonly means “in/during” for seasons: en invierno. Durante (el) invierno is also fine and a bit more explicit. Por invierno is not idiomatic for this meaning.

Could I say Lo uso mucho en invierno instead of a menudo?

Yes, but nuance changes: a menudo = often (frequency). Mucho = a lot (quantity/intensity). Both are natural, just slightly different focuses.