Seguramente mi amigo llegará tarde por el tráfico.

In this sentence, seguramente most naturally means probably, most likely, or surely in the sense of strong likelihood.

So the idea is not absolute certainty, but a confident guess:

• Seguramente mi amigo llegará tarde = My friend will probably arrive late

In English, surely can sometimes sound stronger than the Spanish word does in everyday use. That is why probably is often the best translation.

Llegará is the simple future of llegar, meaning he will arrive.

It is formed like this:

• infinitive: llegar
• future ending for él/ella/usted: -á
• result: llegará

This tense is used because the arrival has not happened yet. The speaker is talking about what will happen later.

Yes. Mi amigo va a llegar tarde por el tráfico is completely natural.

Both are correct:

• llegará tarde = will arrive late
• va a llegar tarde = is going to arrive late

In many everyday situations, Spanish speakers often use ir a + infinitive for near-future events. The version with llegará can sound a little more neutral or slightly more formal, but both are common.

Because Spanish normally uses tarde as an adverb in this expression.

• llegar tarde = to arrive late

This is the standard, natural way to say it.
Tardemente exists in theory as an adverb form, but it is not what people normally use for being late. It would sound unnatural here.

Por is used here to express cause or reason.

So:

• por el tráfico = because of the traffic / due to traffic

This is different from para, which usually expresses purpose, goal, or destination.

For example:

• Estudio para aprender = I study in order to learn
• Llega tarde por el tráfico = He arrives late because of traffic

Spanish often uses the definite article where English might not.

So el tráfico is the normal way to say traffic in this kind of sentence.
Without the article, por tráfico, it would sound incomplete or unnatural in standard Spanish.

This is very common with general nouns in Spanish:

• me gusta el café
• odio el ruido
• llegará tarde por el tráfico

Yes. Spanish adverbs are often flexible in position.

All of these are possible:

• Seguramente mi amigo llegará tarde por el tráfico
• Mi amigo seguramente llegará tarde por el tráfico
• Mi amigo llegará tarde por el tráfico, seguramente

The first two are the most neutral.
Starting with seguramente gives immediate emphasis to the idea of probability.

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

Here:

• llegará already tells you it is he/she/you formal will arrive

And the subject is also stated explicitly as mi amigo, so adding él would usually be unnecessary:

• Mi amigo llegará tarde = natural
• Él, mi amigo, llegará tarde = possible, but marked or emphatic

Spanish does this much more than English.

Yes, amigo is masculine singular, so it normally refers to a male friend.

If the friend were female, you would say:

• mi amiga

Since mi does not change for masculine or feminine singular, the difference appears in the noun:

• mi amigo
• mi amiga

The accent marks show where the stress goes.

• llegará is stressed on the last syllable: lle-ga-RÁ
• tráfico is stressed on the first syllable: TRÁ-fi-co

They also follow standard Spanish spelling rules:

• future forms like llegará, comerá, vivirá take written accents
• tráfico needs an accent because otherwise the stress would fall in the wrong place

Accent marks in Spanish are not optional; they are part of correct spelling.

Tráfico is perfectly correct and natural. It means traffic in a general sense.

In Spain, people also very often say atasco when they mean a traffic jam specifically.

So these are both natural:

• Llegará tarde por el tráfico = He will arrive late because of traffic
• Llegará tarde por el atasco = He will arrive late because of the traffic jam

If you want to sound a bit more specific about cars being stuck, atasco is very common in Spain.