Learn Spanish (Spain): La calle principal está cerrada hoy por la manifestación. - The main street is closed today because of the demonstration.

Por is used here to express the cause or reason, translating to 'because of' or 'due to' when followed directly by a noun (like la manifestación). If you wanted to use porque (because), you would need a full verb phrase after it, such as porque hay una manifestación (because there is a demonstration). You cannot use para here because para is for purposes, goals, or destinations, not causes.

Why do we use está cerrada instead of es cerrada?

We use the verb estar with a past participle (like cerrada) to describe a current state or condition that is the result of an action. The street is currently in a 'closed state'. If you used es cerrada, it would describe the passive action of the street being closed by someone on a regular basis.

Does the adjective principal change depending on whether the noun is masculine or feminine?

No, adjectives that end in a consonant, such as principal, do not change to match the gender of the noun. It remains principal whether it describes a feminine noun like la calle or a masculine noun like el edificio. You only need to add -es if the noun is plural (principales).

Could the time word hoy be placed somewhere else in this sentence?