Этот заказ слишком тяжёлый, чтобы нести его одной.

Here заказ means an order in the shopping/delivery/restaurant sense — something that has been ordered.

It does not mean a command. For command/order in that sense, Russian normally uses приказ.

So in this sentence, заказ is a physical thing that can be heavy and carried.

Because they agree with заказ.

• заказ is masculine
• it is singular
• here it is in the nominative case, because it is the subject

So the words describing it also take masculine singular nominative forms:

• этот заказ
• тяжёлый заказ
• Этот заказ тяжёлый

This is a very common Russian pattern meaning too ... to ...

So:

• слишком тяжёлый = too heavy
• чтобы нести его одной = to carry it alone

Together, the structure means that the order has such a high degree of heaviness that carrying it alone is not realistic/possible.

A few similar examples:

• Он слишком устал, чтобы работать. = He is too tired to work.
• Коробка слишком большая, чтобы пройти в дверь. = The box is too big to fit through the door.

Because чтобы introduces a subordinate clause.

Russian normally puts a comma before чтобы, just as English often separates too X to Y ideas into two parts, even if English punctuation looks different.

So the sentence is divided like this:

• Этот заказ слишком тяжёлый,
• чтобы нести его одной.

Because нести is used for carrying something in one specific act/direction, while носить usually suggests repeated, habitual, or general carrying.

Here the meaning is about one concrete carrying situation: this order is too heavy for someone to carry alone. So нести is the natural choice.

Compare:

• нести = to carry right now / in one particular instance
• носить = to carry regularly / around / habitually

Also, нести is imperfective, which fits well here because the sentence focuses on the action itself, not on completing it.

If you wanted to emphasize carrying it all the way to a destination, you might hear something like донести instead.

Его means it and refers back to заказ.

The verb нести needs a direct object here: carry what? Carry it.

So:

• нести его = to carry it

A detail that often surprises learners: for personal pronouns, the accusative form of masculine/neuter он/оно is его, even when the thing is inanimate.

So although заказ is inanimate, его is still correct.

Because this is an infinitive construction, and the understood person doing the carrying is treated differently than with a normal finite verb.

In a sentence with a normal verb, you would say:

• Я несу его одна. = I am carrying it alone.
  Here одна is nominative, matching я.

But after an infinitive like нести, Russian often uses an implied dative person:

• (мне) нести его одной

So одной means alone / by myself, and it shows that the understood carrier is female.

This is one of the key things English speakers notice: English alone does not show gender, but Russian does here.

Yes.

Одной is feminine, so it tells us that the person meant as alone / by herself is female.

If the person were male, you would normally say:

• Этот заказ слишком тяжёлый, чтобы нести его одному.

If you wanted to make the person explicit, you could say:

• Этот заказ слишком тяжёлый, чтобы мне нести его одной.
• Этот заказ слишком тяжёлый, чтобы ему нести его одному.

Yes, it can be added.

Russian often leaves out the understood person when the context is clear. So:

• Этот заказ слишком тяжёлый, чтобы нести его одной.

can be expanded to:

• Этот заказ слишком тяжёлый, чтобы мне нести его одной.

Adding мне makes the sentence more explicit: too heavy for me to carry alone.

Without мне, the sentence is still natural if the context already makes it obvious who is supposed to carry it.

Yes, that is possible.

Russian word order is flexible, and both versions are grammatical:

• чтобы нести его одной
• чтобы его нести одной

The version in your sentence, нести его, is neutral and straightforward.

Putting его earlier can give it a little more emphasis, as if highlighting it specifically. But in many contexts, the difference is small.