Por si fuera poco, la tarlatana sigue húmeda.

It’s a set expression meaning something like:

• as if that weren’t enough
• to make matters worse
• on top of that

It introduces an extra problem or annoyance after other problems have already been mentioned.

So Por si fuera poco, la tarlatana sigue húmeda means that the cloth still being damp is one more bad thing added to the situation.

Because por si fuera poco uses the imperfect subjunctive of ser: fuera.

This expression is not stating a plain fact like it was little or it is little. Instead, it has a more hypothetical or idiomatic sense, like:

• as if that were not enough

That kind of idea often triggers the subjunctive in Spanish.

So here:

• fuera = imperfect subjunctive of ser
• the phrase is idiomatic, and native speakers learn it as a chunk: por si fuera poco

It is very commonly used as a fixed expression, and learners are best off memorising it as one piece.

You may also see close variants, such as:

• por si eso fuera poco
• como si fuera poco

These mean roughly the same thing.

But por si fuera poco is the standard, very natural version. Even though you can analyse its grammar, in practice it often works like an idiom.

Seguir + adjective means to remain or to still be in a certain state.

So:

• sigue húmeda = it is still damp / it remains damp

The verb seguir literally often means to continue, but with an adjective it commonly expresses that a condition has not changed.

Examples:

• sigue abierta = it’s still open
• sigue enfermo = he’s still ill
• siguen rotas = they’re still broken

So here the sentence is saying that the tarlatan has not dried yet.

Because húmeda agrees with la tarlatana, which is a feminine singular noun.

In Spanish, adjectives usually agree in gender and number with the noun they describe:

• la tarlatana húmeda
• el paño húmedo
• las telas húmedas

Since tarlatana is feminine singular, the adjective must also be feminine singular: húmeda.

Tarlatana is a specific type of fabric: tarlatan, a light, open-weave cloth, often sized or stiffened depending on use.

It is not an everyday word for many learners, and even many native speakers would recognise it as a specialised textile term rather than very common daily vocabulary.

So if this word feels unfamiliar, that’s normal.

Because por si fuera poco functions as an introductory comment on the whole sentence.

It is similar to English phrases like:

• to make matters worse, ...
• as if that weren’t enough, ...

The comma helps separate that introductory expression from the main clause:

• Por si fuera poco, la tarlatana sigue húmeda.

You will usually see it written this way.

Yes, in many contexts that would be very similar:

• la tarlatana sigue húmeda
• la tarlatana todavía está húmeda

Both can mean the tarlatan is still damp.

There is a slight nuance:

• sigue húmeda emphasises that the state continues
• todavía está húmeda simply says it is still damp

In many real situations, the difference is small, and both are natural.

It looks the same, but here it is not from ir.

Spanish has some forms that are identical in spelling but come from different verbs. In this sentence:

• fuera = imperfect subjunctive of ser
  and also historically of ir, depending on context

But in por si fuera poco, it clearly comes from ser, because the meaning is:

• as if it were not enough

So the context tells you which verb is involved.

A little, yes, but the original order is the most natural.

Standard version:

• Por si fuera poco, la tarlatana sigue húmeda.

You could sometimes move things around for emphasis, but it may sound less neutral. For example:

• La tarlatana sigue húmeda, por si fuera poco.

That sounds more like an afterthought.

So for normal use, it’s best to keep:

• introductory phrase first
• then the main clause

It is pronounced with the stress on the first syllable: HÚ-me-da.

The written accent mark shows where the stress goes:

• húmeda

Without the accent mark, the normal stress rules of Spanish would suggest a different pronunciation, so the accent is needed.

Also, the h is silent, so it sounds roughly like:

• OO-meh-dah

with Spanish vowel sounds, not English ones.

It is mostly neutral, though one word makes it feel a bit specialised:

• por si fuera poco = very normal, common expression
• sigue húmeda = normal everyday grammar
• tarlatana = specialised vocabulary

So the sentence itself is not especially formal, but it may feel a bit technical or literary because tarlatana is not a common everyday noun.