Пилот сказал, что посадка будет мягкой.

Here что means that. It introduces a subordinate clause:

Пилот сказал, что ... = The pilot said that ...

Russian что can also mean what in other contexts, so learners often get confused. In this sentence, because it comes after сказал, it is a conjunction meaning that.

A useful extra note: although it is spelled что, it is usually pronounced што in standard speech.

Because что introduces a subordinate clause, and Russian normally separates that clause with a comma:

Пилот сказал, что посадка будет мягкой.

This is standard Russian punctuation. English often does not use a comma before that, but Russian does in this structure.

Сказал is the natural choice when you mean a single completed act of saying.

• сказал = said, told, one completed statement
• говорил = was saying, used to say, or spoke with more focus on the process or repeated action

So here, the pilot made one statement, and Russian normally uses сказал.

That is the normal marker of the Russian past tense.

In the past tense, Russian verbs also agree in gender and number:

• сказал = masculine singular
• сказала = feminine singular
• сказало = neuter singular
• сказали = plural

Since пилот is masculine singular, the verb is сказал.

Because Russian does not usually do the same kind of tense backshifting that English does in reported speech.

English:

• The pilot said that the landing would be soft.

Russian:

• Пилот сказал, что посадка будет мягкой.

Russian keeps будет because the landing is still in the future relative to the moment of speaking. So this is completely normal.

Because the subject of the subordinate clause is посадка, and посадка is singular.

• посадка будет = the landing will be
• посадки будут = the landings will be

So будет is simply the correct singular form.

Because after быть in the past or future, Russian often uses the predicate adjective in the instrumental case.

So:

• посадка будет мягкой
• встреча была короткой
• задача будет трудной

Here мягкой is the feminine singular instrumental form, matching посадка.

A helpful comparison:

• Посадка мягкая. = present-time statement, no быть
• Посадка будет мягкой. = future statement

For learners, мягкой is the safest and most standard choice in this sentence.

Because мягкой is an adjective, while мягко is usually an adverb.

Here the word describes the noun посадка, so Russian uses an adjective that agrees with it:

• посадка будет мягкой = the landing will be soft/smooth

If you used мягко, you would be describing how something happens, not describing the noun itself:

• Самолёт сядет мягко. = The plane will land softly.

So in this sentence, мягкой is correct.

Yes, Russian word order is fairly flexible, but the given order is the most neutral and natural:

Пилот сказал, что посадка будет мягкой.

Russian can move words around for emphasis, contrast, or style, but that changes the feel of the sentence. For example:

• Пилот сказал, что мягкой будет посадка.

This is possible, but it sounds more marked and less neutral. For everyday use, the original word order is best.

In this sentence, посадка means landing in the aviation sense.

But посадка can have other meanings in different contexts, such as:

• planting
• seating
• placement

The word is understood as landing here because of the context: пилот makes the aviation meaning obvious.

The stresses are:

пило́т сказа́л, что поса́дка бу́дет мя́гкой

A few pronunciation notes:

• что is usually pronounced што
• бу́дет has stress on the first syllable
• мя́гкой has stress on мя́-

So a natural pronunciation is close to:

pee-LOT ska-ZAL, shto pa-SAT-ka BOO-det MYAG-kay