Cada columna del gráfico explica una idea diferente, y yo la marco con un subrayador amarillo.

Because cada is followed by a singular noun in Spanish.

• cada columna = each column
• not cada columnas

This is different from English, where learners sometimes expect a plural because the idea is plural overall. In Spanish, cada treats the items one by one.

Compare:

• Cada estudiante tiene un libro.
• Cada día aprendo algo nuevo.

Because de + el normally contracts to del.

So:

• de + el gráfico → del gráfico

This is a standard contraction in Spanish, just like:

• a + el → al

Examples:

• la portada del libro
• voy al museo

A useful reminder: this only happens with the article el, not with the pronoun él.

It can cover both, depending on context. In everyday Spanish, gráfico is often a general word for a visual representation of data.

Here, because the sentence talks about columnas, it suggests a column chart / bar chart type of graphic.

Useful related terms:

• gráfico = graph / chart / graphic
• gráfico de barras = bar chart
• gráfico de columnas = column chart
• gráfica can also mean graph, especially in some contexts, but gráfico is very common as a broad term

So a learner should not assume there is always a neat one-to-one match with English.

Because in Spanish, adjectives often come after the noun by default.

So:

• una idea diferente = a different idea

This is the most neutral, standard order.

If you move an adjective before the noun in Spanish, it can sometimes sound more literary, emphatic, or slightly change the nuance. For basic learning, it is safest to remember that descriptive adjectives usually go after the noun unless there is a special reason not to.

La is a direct object pronoun, meaning it here.

It is feminine singular, so grammatically it could refer to:

• la columna
• la idea

Since both are feminine singular, the sentence by itself is grammatically ambiguous. Normally, context tells you what la refers to.

So the important grammar point is:

• la = feminine singular direct object
• lo would be masculine singular instead

Examples:

• el gráfico → lo
• la columna → la

Because the pronoun must agree with the noun it replaces in gender and number.

• la = feminine singular
• lo = masculine singular

In this sentence, the possible referents are feminine singular nouns:

• columna
• idea

So la is the correct form.

That is true: Spanish often omits subject pronouns because the verb ending already shows the person.

So you could also say:

• ... y la marco con un subrayador amarillo.

Adding yo gives emphasis, contrast, or a clearer switch of subject.

Here it can sound like:

• ... and I mark it... possibly contrasting with what someone else does.

So yo is not required for grammar, but it is perfectly natural if the speaker wants emphasis.

Because marcar is a broader verb meaning to mark, to highlight, or to indicate.

• marcar = general idea of marking something
• subrayar = to underline, and by extension sometimes to highlight or emphasize

If you are literally drawing a line under text, subrayar is very natural. If you are more generally highlighting or marking something, marcar works well.

In this sentence, marco is a flexible choice, especially if the speaker is just indicating that they mark the item using a yellow highlighter.

A subrayador is a highlighter or marking pen used to underline or highlight text.

In Spain, learners may also hear:

• marcador fluorescente
• rotulador fluorescente

So subrayador amarillo is basically a yellow highlighter.

The word comes from subrayar = to underline.

Because amarillo describes subrayador, and subrayador is masculine.

So the agreement is:

• un subrayador amarillo

Not:

• un subrayador amarilla

Even though the object being marked might be feminine (la columna or la idea), the adjective amarillo is attached to subrayador, not to that object.

Spanish usually does not use a comma before y when simply joining words or short phrases, but with two full clauses, a comma can appear, especially when:

• the clauses are a bit longer
• there is a change of subject
• the writer wants a slight pause for clarity

Here we have:

• Cada columna del gráfico explica una idea diferente
• yo la marco con un subrayador amarillo

Since the second clause introduces yo, the comma is understandable and acceptable. Many writers might also omit it, depending on style.

So this comma is not strange, but it is also not something you should add mechanically before every y.