Nosotros pintamos una pancarta verde para apoyar la manifestación ecológica contra el plástico.

You can absolutely drop nosotros and just say:

• Pintamos una pancarta verde para apoyar…

In Spanish, the verb ending (-amos) already shows who the subject is (we), so the subject pronoun (nosotros) is usually omitted.

You keep nosotros when you:

• Want to emphasize we (as opposed to someone else):
  Nosotros pintamos la pancarta, no ellos.
• Need contrast or clarity in context.

In everyday Latin American Spanish, the shorter form without nosotros is very common.

Pintamos (with nosotros) can be either:

• Present: we paint
• Preterite (simple past): we painted

They look the same in spelling and pronunciation.

You know the tense from context:

• Present: talking about a habit or something happening now
  Siempre pintamos pancartas para las marchas. – We always paint banners…
• Past: a completed action in the past
  Ayer pintamos una pancarta verde… – Yesterday we painted…

Your sentence by itself is ambiguous; in real conversation, the surrounding sentences (or a time word like ayer, hoy, siempre) tell you whether it’s present or past.

In Spanish, every noun has a grammatical gender. Pancarta is a feminine noun, so it normally takes:

• la (the)
• una (a)

So you say:

• una pancarta – a banner
• la pancarta – the banner

There is no logical reason beyond usage; you simply learn each noun’s gender:

• el cartel (masculine) – poster, sign
• la pancarta (feminine) – usually a big banner used in marches/demonstrations

In Latin American Spanish:

• pancarta
  A banner/sign, usually large, often held up in protests, marches, or demonstrations. Typically made of cloth or large pieces of paper/cardboard with a slogan.

• cartel
  A poster; can be for advertising, events, information, or decoration. Not specifically for protests.

• letrero
  A sign, usually more permanent, like a shop sign or street sign.

So in a protest context, pancarta is the most natural word for the kind of big banner you carry to show support or opposition.

The normal word order in Spanish is:

• noun + adjective

So:

• una pancarta verde – a green banner
• una casa grande – a big house
• un coche nuevo – a new car

Some adjectives can go before the noun (especially very common ones like bueno, malo, gran(de), nuevo, viejo), and sometimes that changes the nuance. But with colors and most descriptive adjectives, putting them after the noun is the standard, neutral pattern:

• ✅ una pancarta verde
• ❌ una verde pancarta (grammatically possible, but sounds poetic/emphatic, not neutral)

Adjectives that end in -e in their basic form usually do not change for gender, only for number:

• verde (green)
  
  • una pancarta verde – feminine singular
  • un cartel verde – masculine singular
  • dos pancartas verdes – feminine plural
  • dos carteles verdes – masculine plural

So:

• Gender: same form (verde) for masculine and feminine.
• Number: add -s in the plural (verdes).

Other adjective types do change for gender, e.g.:

• rojo/roja, ecológico/ecológica, etc.

Para + infinitive is the most common way to express purpose in Spanish, similar to in order to / to in English.

• Pintamos una pancarta verde para apoyar la manifestación…
  → We painted a green banner to support the demonstration…

Structure:

• para + [infinitive] = for the purpose of [doing something]

If you said:

• Pintamos una pancarta verde y apoyamos la manifestación…
  → We painted a banner and we supported the demonstration…
  (Just two actions; you lose the clear “purpose” idea.)

Using para apoyar clearly links the painting of the banner to its goal: supporting the ecological demonstration.

In Spanish, you use the personal a mainly with:

• People: Veo a María.
• Pets/animals treated like people: Quiero a mi perro.

For things, events, or abstract concepts you usually don’t use a:

• apoyar la manifestación – support the demonstration (event)
• apoyar el movimiento – support the movement
• apoyar la idea – support the idea

So apoyar la manifestación is correct because la manifestación is an event, not a person.

Because manifestación is a feminine noun:

• la manifestación – the demonstration

The adjective must agree in gender and number with the noun it describes:

• ecológico (masculine) → el movimiento ecológico
• ecológica (feminine) → la manifestación ecológica

So:

• la manifestación ecológica (feminine noun + feminine adjective)
• las manifestaciones ecológicas (both plural and feminine)

They’re related but not identical:

• ecológica
  Literally “ecological”; here it means the demonstration is about ecology / the environment. Very natural in this sentence.

• ambiental
  “Environmental”; focuses on the environment in general.
  manifestación ambiental – an environmental demonstration (similar meaning; also fine).

• ecologista
  Often used to describe people or things with an environmentalist ideology:

  • un partido ecologista – a green/environmentalist party
  • una marcha ecologista – a march organized by or associated with environmentalists

In your sentence, manifestación ecológica is standard and clearly understood as an environmentally focused protest.

Contra means against, expressing opposition:

• contra el plástico – against plastic

En contra de is a slightly longer form; in many cases it’s interchangeable with contra, but it can sound a bit more explicit or emphatic:

• una manifestación contra el plástico
• una manifestación en contra del plástico

Both are correct and common. In your sentence, contra is shorter and perfectly natural.

In Spanish, when you talk about something in a general sense (a whole category, like “plastic” in general), you often use the definite article:

• El plástico es un problema. – Plastic is a problem.
• Estamos en contra del plástico. – We are against plastic.

So:

• contra el plástico (or en contra del plástico) is the usual, natural way.

Saying contra plástico without the article is possible in a few set expressions or headlines, but it sounds less natural in a full sentence like yours. The article el makes it sound like normal, standard Spanish.

The main order is fairly fixed, but some small variations are possible. For example:

• Pintamos una pancarta verde nosotros para apoyar…
  → Grammatically possible, but nosotros at the end sounds unusual or emphatic.

• Pintamos nosotros una pancarta verde para apoyar…
  → Also possible; nosotros is emphasized (“we painted…”).

• Pintamos una pancarta verde para apoyar la manifestación ecológica contra el plástico.
  → Most natural, especially if you drop nosotros.

You generally keep:

1. Subject (often omitted)
2. Verb (pintamos)
3. Direct object (una pancarta verde)
4. Purpose (para apoyar…)
5. Additional details (ecológica contra el plástico)

Reordering too much can sound poetic, overly emphatic, or unnatural in everyday Latin American Spanish.