La ropa no puede ni desteñir ni encoger.

Because ropa is usually an uncountable/collective noun in Spanish, like clothing in English. Even if you are talking about several garments, Spanish normally says la ropa.

So:

• la ropa = clothing / clothes in general
• las ropas is uncommon and usually sounds very specific, literary, or old-fashioned

That is why the sentence uses la ropa.

Because the subject is la ropa, which is grammatically singular.

Even though la ropa may refer to many items, Spanish treats it as one singular noun, so the verb must also be singular:

• La ropa puede...
• not La ropa pueden...

This is similar to English The clothing is..., not The clothing are...

Ni...ni means neither...nor.

So:

• ni desteñir ni encoger = neither fade nor shrink

Because the sentence already has no puede, the idea in natural English is often simply:

• The clothing can’t fade or shrink

But the Spanish structure is explicitly negative and balanced: not... neither... nor...

Spanish often repeats ni before each item in a negative pair:

• ni X ni Y = neither X nor Y

So:

• ni desteñir ni encoger

This gives a neat, parallel structure.

You may also hear simpler patterns after a negative, such as:

• La ropa no puede desteñir ni encoger

That is also understandable and natural. The version with ni before both verbs sounds a little more formal or more carefully structured.

Because after poder (can / be able to), Spanish normally uses an infinitive.

The pattern is:

• poder + infinitive

Examples:

• puede venir = can come
• puede lavar = can wash
• puede encoger = can shrink

So here:

• no puede ni desteñir ni encoger

means that the clothing cannot / must not fade or shrink.

For clothes or fabric, desteñir means to lose colour, fade, or sometimes run/bleed dye.

Depending on context, it can mean:

• the colour becomes weaker
• the dye comes out in the wash
• the item stains other clothes with its colour

So in a laundry or fabric context, desteñir is about colour not staying stable.

Here encoger means to shrink.

When talking about clothes, it means the fabric becomes smaller, usually after washing or drying.

For example:

• Esta camiseta encoge al lavarla.
  = This T-shirt shrinks when you wash it.

So no puede... encoger means the clothing should not get smaller.

It can mean either, depending on context.

Literally, no puede means cannot. But in product descriptions, labels, or technical statements, it often has the practical sense of:

• must not
• should not
• is not supposed to

So in a textile or clothing context, the sentence often means something like:

• The clothing must not fade or shrink
• The clothing should not fade or shrink

not necessarily that it is physically impossible, but that this should not happen.

Because encoger can already work by itself when talking about clothes or fabric shrinking.

For example:

• La lana encoge. = Wool shrinks.
• Esta camisa encoge. = This shirt shrinks.

Spanish also has encogerse, but here the simple verb encoger is completely normal and idiomatic. In this sentence, encoger is the standard choice.

It usually refers to clothing in general or the garment(s) being discussed, not necessarily just one item.

So the sentence could appear in contexts like:

• a product description
• a care instruction
• a fabric specification

In other words, la ropa here is a general category: the clothing / the garment / the clothes.