How to Say "to lengthen" in Spanish
The most common Spanish word for “to lengthen” is “alargar” — use 'alargar' when referring to making a physical object, like clothing or a piece of rope, longer..
alargar
/ah-lar-GAR//alaɾˈɡaɾ/

Examples
Necesito alargar la manga de mi camisa.
I need to lengthen the sleeve of my shirt.
Tengo que alargar estos pantalones porque me quedan cortos.
I have to lengthen these pants because they are too short for me.
Vamos a alargar la cuerda para que llegue al suelo.
We are going to make the rope longer so it reaches the ground.
Si quieres alargar el paso, llegaremos más rápido.
If you want to lengthen your stride (walk faster), we will arrive sooner.
The 'G' to 'GU' Spelling Change
In the 'yo' form of the past tense (preterite) and all forms of the 'possibility' mood (subjunctive), the 'g' changes to 'gu' to keep the sound hard, like in 'go' or 'gift'.
Physicality
When used physically, it literally means adding material or stretching something to increase its measurement.
Spelling in the Past
Mistake: “Yo alargé mucho la cuerda.”
Correction: Yo alargué mucho la cuerda. (You must add the 'u' after 'g' when it's followed by an 'e' to keep the pronunciation correct).
prolongar
/pro-lon-GAR//pɾolonˈɡaɾ/

Examples
Van a prolongar el contrato por seis meses más.
They are going to extend the contract for six more months.
Van a prolongar la línea del metro hasta el aeropuerto.
They are going to extend the subway line to the airport.
El arquitecto quiere prolongar el balcón.
The architect wants to extend the balcony.
Si prolongas esta línea, verás que cruza el círculo.
If you extend this line, you will see it crosses the circle.
Talking About Distance
In this sense, the word describes physical growth or addition to something that already exists, like a road or a building.
Physical Objects vs. Abstract Extensions
Related Translations
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