Mi sobrina dibuja en una tableta pequeña cuando viaja en tren.

Yes. Mi means my.

In Spanish, mi is a possessive adjective, and it goes before the noun:

• mi sobrina = my niece
• mi hermano = my brother
• mi tableta = my tablet

Unlike English, mi does not change for masculine or feminine singular nouns:

• mi sobrina
• mi sobrino

But it does change in the plural:

• mis sobrinas
• mis sobrinos

Sobrina means niece. It is a feminine noun.

Compare:

• sobrina = niece
• sobrino = nephew

Because it is feminine singular, any adjective describing it would also need to be feminine singular. In this sentence there is no adjective describing sobrina, but the noun itself is feminine.

Dibuja is the third-person singular present indicative form of dibujar (to draw).

So:

• yo dibujo = I draw
• tú dibujas = you draw
• él / ella dibuja = he / she draws

Here it matches mi sobrina, which is she:

• Mi sobrina dibuja = My niece draws / is drawing

In Spanish, the present tense can often translate as either:

• draws
• is drawing

The context tells you which sounds more natural in English.

Because Spanish often omits subject pronouns when they are not needed.

The verb ending already shows the person:

• dibuja = he/she draws

So Mi sobrina dibuja is perfectly complete without ella.

You could say Ella dibuja, but Spanish usually avoids subject pronouns unless you want emphasis, contrast, or clarity.

Here en means something like on in English.

So:

• dibujar en una tableta = to draw on a tablet

Spanish often uses en where English uses on, especially with surfaces, devices, places, or means.

Examples:

• Escribe en el ordenador = He/She writes on the computer
• Lo vi en la televisión = I saw it on television
• Dibuja en papel = He/She draws on paper

Because in Spanish, adjectives usually come after the noun.

So the normal order is:

• una tableta pequeña = a small tablet

This is the most neutral, everyday order.

If you put the adjective before the noun, it can sound more literary, emotional, or stylistically marked:

• una pequeña tableta

That is possible, but it feels less neutral here.

Also, the adjective agrees with the noun:

• tableta is feminine singular
• so the adjective is pequeña, not pequeño

Because it must agree with tableta, which is feminine singular.

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

• un libro pequeño = a small book
• una tableta pequeña = a small tablet
• unos libros pequeños = some small books
• unas tabletas pequeñas = some small tablets

So pequeña is the feminine singular form of pequeño.

In this context, yes, that is the most natural meaning.

But tableta can mean different things depending on context, for example:

• a tablet device
• a chocolate bar
• sometimes a slab or block of something

Because the sentence says dibuja en una tableta pequeña, the meaning is clearly an electronic tablet or drawing tablet.

Spanish often uses the simple present for things that happen regularly, habitually, or as part of a general pattern.

So:

• cuando viaja en tren = when she travels by train

This suggests something she usually does when that situation happens.

If you said cuando está viajando en tren, it would focus more on the action as in progress at a specific moment. That is grammatically possible in some contexts, but it is less natural for a general habit.

So here the simple present is the best choice.

Because Spanish often uses en + transport without an article when talking about the means of transport in a general way.

So:

• viajar en tren = to travel by train
• ir en coche = to go by car
• venir en autobús = to come by bus

This works like English by train, even though Spanish uses en.

If you add an article, the meaning can become more specific:

• en el tren = on the train / in the train

So:

• viaja en tren = she travels by train
• está en el tren = she is on the train

Yes. The Spanish present tense often expresses a habit or repeated action.

So this sentence naturally suggests:

• whenever she travels by train, she draws on a small tablet

That is one reason cuando viaja en tren sounds very natural here. It describes a recurring situation.

Yes, Spanish word order is somewhat flexible.

The given sentence is neutral and very natural:

• Mi sobrina dibuja en una tableta pequeña cuando viaja en tren.

But you could also say:

• Cuando viaja en tren, mi sobrina dibuja en una tableta pequeña.

This puts more focus first on the circumstance when she travels by train.

Both are correct. The original version is just a straightforward, standard order.

A simple pronunciation guide would be:

Mi so-BREE-na dee-BOO-ha en OO-na ta-BLE-ta pe-KE-ña KWAN-do BYA-ha en tren

A few useful Spain-specific notes:

• z and soft c are pronounced with the th sound of think in much of Spain, but there is no z/c in this sentence.
• j in dibuja and viaja is a strong throaty sound, like the ch in Scottish loch
• ll is often pronounced like y
• ñ in pequeña sounds like ny in canyon

So:

• dibuja ≈ dee-BOO-ha
• viaja ≈ BYA-ha
• pequeña ≈ pe-KE-nya

A quick breakdown is:

• Mi sobrina = subject
• dibuja = main verb
• en una tableta pequeña = prepositional phrase telling you where/on what she draws
• cuando viaja en tren = time clause telling you when she does it

So the structure is:

subject + verb + extra information + time clause

That is a very common Spanish sentence pattern.