Learn Spanish (Spain): Veo setenta árboles en el parque.

Setenta means seventy (70), and sesenta means sixty (60). A helpful trick is to look at the third letter: seTenta has a 't' like siete (seven), while seSenta has an 's' like seis (six).

Do I need to change setenta to match the masculine word árboles?

No, the tens numbers (30 to 90) never change their endings to match masculine or feminine nouns. It is always setenta, whether you are counting árboles (masculine) or casas (feminine).

Why is there no word for 'I' before veo?

In Spanish, the verb ending usually tells you who is doing the action. Veo specifically means 'I see' (from the verb ver). You can say yo veo, but native speakers almost always drop the yo unless they need to emphasize who is looking.

Why does the plural word árboles have an accent mark on the 'a'?

The singular word is árbol. When you add -es to make it plural (árboles), the spoken stress stays on the 'a'. However, that 'a' is now three syllables from the end of the word. In Spanish, any word with the voice stress that far back must have a written accent mark to show you how to pronounce it.