После тяжёлого дня мне хочется делать что угодно, только не начинать спор.

Because после always requires the genitive case.

So:

• день → дня in the genitive singular
• the adjective must agree with the noun, so тяжёлый → тяжёлого

That is why you get:

• после тяжёлого дня = after a hard/tiring day

This is a very common pattern:

• после урока = after the lesson
• после работы = after work
• после долгого разговора = after a long conversation

Мне хочется is a very common Russian way to say I feel like... or I have the urge/desire to...

Literally, it works like an impersonal construction:

• мне = to me
• хочется = it is wanted / it feels desired

So the idea is not a direct, active I want, but rather I feel like.

Compare:

• Я хочу спать. = I want to sleep.
• Мне хочется спать. = I feel like sleeping.

In many contexts, мне хочется sounds softer, more emotional, and more natural when talking about a passing feeling.

In this sentence, мне хочется fits very well because it describes a mood after a tiring day.

Хочется comes from хотеться, which is a reflexive/impersonal verb meaning to feel like, to want in the sense of an inner urge.

A few useful points:

• хотеть = to want
• хотеться = to feel like, to have the desire to

The ending -ся often appears in verbs that are used impersonally or describe a state happening to someone rather than something they actively do.

So:

• Я хочу = I want
• Мне хочется = I feel like / I have a desire to

Also, хочется is in the 3rd person singular, because impersonal verbs often appear in that form.

Because in impersonal constructions like мне хочется, the person experiencing the feeling is put in the dative.

So:

• я → nominative = I
• мне → dative = to me

Russian often uses this pattern for physical or emotional states:

• мне холодно = I am cold
• мне скучно = I am bored
• мне не хочется говорить = I don’t feel like talking

So here, мне хочется literally means something like to me, it feels desirable.

Because after хочется, Russian normally uses the infinitive.

So the pattern is:

• мне хочется + infinitive

Examples:

• мне хочется спать = I feel like sleeping
• мне хочется поехать домой = I feel like going home
• мне хочется делать что угодно = I feel like doing anything

The infinitive is used because it expresses the action in a general way, just like in English after want to or feel like.

They are imperfective because the sentence is talking about actions in a general, non-completed sense.

делать

Here, делать что угодно means to do anything at all in a broad, open-ended way.
Using imperfective makes sense because it is not about one completed act, but any kind of activity.

начинать

Начинать is also imperfective because it focuses on the process of beginning, not on a completed single event.

Russian often uses imperfective in these kinds of general desire statements:

• мне хочется читать = I feel like reading
• не хочется начинать разговор = I don’t feel like starting a conversation

Could perfective appear in some contexts? Yes, but it would change the nuance. For example:

• мне хочется сделать что-нибудь would sound more like I feel like doing some particular thing / getting something done
• не хочется начать спор would be much less natural here; начинать спор is the normal choice

So in this sentence, the imperfective verbs are the natural option.

Что угодно means anything, whatever, or more literally whatever you like / anything at all, depending on context.

In this sentence, it means:

• anything at all

The expression is built from:

• что = what
• угодно = pleasing, desired, acceptable

Together, что угодно is a fixed expression.

Examples:

• делай что угодно = do whatever you want
• я съем что угодно = I’ll eat anything
• мне хочется делать что угодно, только не работать = I feel like doing anything but working

It is best learned as a chunk: что угодно.

Here только не is part of the very common pattern:

• что угодно, только не X

This means:

• anything but X
• anything except X

So in this sentence:

• делать что угодно, только не начинать спор
• to do anything, just not start an argument
• more naturally: to do anything but start an argument

So the не is not simply negating the whole sentence. It is specifically excluding one option.

More examples:

• Я съем что угодно, только не рыбу. = I’ll eat anything but fish.
• Он пойдёт куда угодно, только не домой. = He’ll go anywhere but home.

Because the sentence has a contrast structure:

• что угодно, только не...

The comma marks the break before the excluded option. It helps show the meaning:

• anything ..., just not ...
• anything but ...

This punctuation is normal and expected in Russian.

Compare:

• Я готов на всё, только не на это.
• Он согласен делать что угодно, только не ждать.

The comma separates the broad possibility from the contrasting exception.

Both are possible, but they are slightly different.

начинать спор

This means to start an argument/dispute.
Here спор is a noun: an argument, a dispute.

начинать спорить

This means to start arguing.
Here спорить is a verb: to argue.

Both can work, but начинать спор presents the argument a little more as a concrete event, while начинать спорить focuses more on the action itself.

So:

• не начинать спор = not to start an argument
• не начинать спорить = not to start arguing

In this sentence, начинать спор is perfectly natural.

Because спор is the direct object of начинать, so it is in the accusative case.

For an inanimate masculine noun, the accusative singular is the same as the nominative singular.

So:

• nominative: спор
• accusative: спор

That is why the form does not change.

Compare:

• начинать разговор = to start a conversation
• начинать спор = to start an argument
• читать журнал = to read a magazine

If the noun were feminine, you would often see a different accusative form:

• начинать ссору = to start a quarrel

The given word order is natural and expressive:

• После тяжёлого дня мне хочется делать что угодно, только не начинать спор.

It starts with the time/background phrase после тяжёлого дня, then gives the experiencer мне, then the feeling хочется, and then the contrast.

Russian word order is flexible, so variants are possible, for example:

• Мне после тяжёлого дня хочется делать что угодно, только не начинать спор.

This is also natural, with slightly different emphasis.

What matters most is keeping the contrast clear:

• что угодно, только не...

That part is the key structure and should normally stay together.

Yes, the sentence is natural.

Спор usually means:

• an argument
• a dispute
• a disagreement

It does not always mean an angry fight; it can range from a disagreement to a heated argument depending on context.

So начинать спор suggests starting some kind of verbal conflict or dispute.

If you wanted something more informal or more obviously emotional, you might choose a different word, for example:

• ссора = quarrel
• конфликт = conflict

But спор works very well here and sounds natural.

Feel like is usually better for мне хочется.

So the closest natural match is:

• After a hard day, I feel like doing anything but starting an argument.

If you say:

• I want to do anything but start an argument

it is understandable, but it sounds a bit stronger and less like a passing mood.

So the distinction is:

• я хочу → I want
• мне хочется → I feel like

In many everyday situations, that difference matters.