Trabaja solamente por la mañana.

In Spanish, subject pronouns are often left out because the verb ending already gives you the person.

Trabaja can mean:

• he works
• she works
• you work (formal singular, usted)

So Spanish does not need to say Él trabaja unless the speaker wants emphasis, contrast, or clarity.

Trabaja is the third person singular present tense of trabajar.

So it corresponds to:

• he works
• she works
• you work (formal singular)

The present tense here can describe:

• a habitual action: He works only in the morning
• a general fact
• sometimes a current situation, depending on context

Yes. Trabaja can also be the informal affirmative command (tú) of trabajar:

• ¡Trabaja! = Work!

So Trabaja solamente por la mañana could mean:

• He/She/You (formal) works only in the morning, or
• Work only in the morning

Usually, context and punctuation make the meaning clear. In isolation, the sentence is ambiguous.

In Spanish, adverbs like solamente often go near the part of the sentence they modify. Here it modifies the idea of working, so placing it after the verb is very natural:

• Trabaja solamente por la mañana

This is similar to saying works only in the morning.

Other placements are also possible, depending on emphasis:

• Solamente trabaja por la mañana
• Trabaja por la mañana solamente

But the original version is very normal and natural.

Yes. In most everyday Spanish, solo and solamente can both mean only.

So you can say:

• Trabaja solo por la mañana
• Trabaja solamente por la mañana

Solamente can sound a little more explicit or slightly more formal, but both are common.

In modern spelling, solo is normally written without an accent when it means only.

In Spain, por la mañana is the standard and most natural way to say in the morning.

You will also hear:

• por la tarde = in the afternoon / evening
• por la noche = at night / in the evening

In some parts of Latin America, en la mañana is also used, but for Spanish from Spain, por la mañana is the best choice here.

Spanish often uses the definite article with parts of the day:

• la mañana = the morning
• la tarde = the afternoon/evening
• la noche = the night

So por la mañana literally looks like by the morning, but the full expression means in the morning.

This is just the normal fixed Spanish pattern.

Usually, por la mañana means in the morning as a time of day, often in a general or habitual sense.

So Trabaja solamente por la mañana normally suggests a routine:

• He/She only works in the mornings
• He/She works only in the morning

If you want to be very explicit about repeated mornings, Spanish can also use:

• solo trabaja por las mañanas = he/she only works in the mornings

Both are possible, but por la mañana is extremely common for habitual meaning.

The Spanish present tense often covers both simple present and present continuous, depending on context.

So trabaja can mean:

• works
• is working

In this sentence, because of solamente por la mañana, the most natural reading is habitual:

• He/She works only in the morning

If you specifically wanted is working right now, Spanish would more often use:

• está trabajando

Yes. Por las mañanas is also correct and often emphasizes repetition or routine:

• Trabaja solamente por las mañanas = He/She only works in the mornings

Compare:

• por la mañana = in the morning
• por las mañanas = in the mornings / every morning kind of idea

In many situations, both sound natural, but por las mañanas can make the habitual sense a little clearer.

Yes, it is completely natural in Spanish from Spain.

A native speaker from Spain would easily understand:

• Trabaja solamente por la mañana

They might also say:

• Solo trabaja por la mañana
• Trabaja solo por la mañana
• Trabaja únicamente por la mañana

All of these are natural, with only small differences in style or emphasis.