El lago está rodeado de montañas.

Spanish has two verbs for “to be”: ser and estar.
• Es (from ser) describes permanent or defining characteristics.
• Está (from estar) describes temporary states or locations.
Here, being “surrounded by mountains” is a state or condition of the lake, so we choose está.

Rodear is a verb meaning “to surround.”
• Rodeado is its past participle, used here as an adjective meaning “surrounded.”
• It must agree in gender and number with the noun it describes: lago (masculine singular) ⇒ rodeado.
• The preposition de introduces what does the surrounding (“surrounded by mountains”).

• With participles used as adjectives (está rodeado de), Spanish uses de to indicate “by” in a general, descriptive sense.
• Por appears in true passive-voice constructions with ser (e.g., fue rodeado por), not with estar plus participle.
• You can use entre to say “between mountains” (location): El lago está entre montañas, but that emphasizes “lying among” rather than “completely encircled.”

• We use the plural because there’s more than one mountain around the lake.
• If only one mountain surrounded it, you would say El lago está rodeado de una montaña, but that sounds unusual—lakes are typically encircled by several peaks.

• In Spanish, general references to countable nouns usually require a definite article: el lago = “the lake.”
• You can drop the article only in very specific poetic or headlines contexts. Normally, you keep el.

Here it functions as an adjective (a so-called “stative” participle), not an active passive.
• Passive voice in Spanish normally uses ser + participle (e.g., fue construido).
• Estar + participle describes condition: está rodeado = “it finds itself surrounded.”

• Montañas is pronounced [mon-TAH-ɲas].
• The ñ (with its tilde) is a separate letter pronounced like “ny” in canyon.
• The stress naturally falls on the second-to-last syllable (TAH), following Spanish stress rules for words ending in a vowel.

Turn the surrounding elements into the subject and use the present of rodear:
• Las montañas rodean el lago.
This means literally “The mountains surround the lake.”

Yes, you can adapt rodear to any tense or person:
• Past imperfect: Las montañas rodeaban el lago (“The mountains used to surround the lake”).
• Future: Las montañas rodearán el lago (“The mountains will surround the lake”).
• First person plural: Rodeamos el lago en kayak (“We surround/encircle the lake by kayaking around it”).
Each form changes the time frame or perspective but uses the same root verb.

Yes. Se encuentra (from encontrarse) is a more formal or literary synonym of estar.
• El lago se encuentra rodeado de montañas conveys the exact same meaning but sounds slightly more formal.
• In everyday conversation, está rodeado de is more common.