kinou ha nagaku arukisugite, hiza to senaka ga sukosi itaku narimasita.

は marks 昨日 as the topic: as for yesterday or yesterday, ...

So 昨日は sets the time frame for the whole sentence. It is very natural in Japanese to make a time expression the topic.

A rough breakdown is:

• 昨日は = yesterday / as for yesterday
• then the speaker explains what happened yesterday

You could also see 昨日 without は in some contexts, but 昨日は feels very natural when introducing what happened on that day.

長い is an i-adjective, and before a verb it usually changes into its adverbial form:

• 長い → 長く

So:

• 長く歩く = to walk for a long time

Here 長く describes how the person walked, so it works like an adverb.

It comes from:

• 歩く = to walk
• 歩き = the stem of 歩く
• すぎる = to exceed, to do too much
• て = links this part to what follows

So:

• 歩きすぎる = to walk too much
• 歩きすぎて、... = having walked too much, ... / because I walked too much, ...

This pattern is very common:

• 食べすぎる = eat too much
• 飲みすぎる = drink too much
• 働きすぎる = work too much

The て-form connects this action to the result in the second half of the sentence.

Here it gives a meaning like:

• I walked too much, and as a result...
• Because I walked too much...

So the sentence flows as:

• 歩きすぎて、ひざと背中が少し痛くなりました。
• I walked too much, and my knees and back became a little painful.

In this sentence, the て-form shows a cause/result relationship.

が marks what experienced the change or what is in the state being described.

Here:

• ひざと背中が = the knees and back
• these are the body parts that became painful

So が is marking the subject of 痛くなりました.

This is very natural with body conditions in Japanese:

• 頭が痛い = my head hurts
• のどが痛い = my throat hurts
• 背中が痛い = my back hurts

Even though English often says my back hurts, Japanese often just names the body part and marks it with が, leaving my understood from context.

Because ひざと背中 is one combined noun phrase:

• ひざと背中 = knees and back

Then が marks that whole phrase:

• ひざと背中が

This is just like saying:

• A and B are the things that became painful

You do not need to say ひざがと背中が.

痛くなりました means became painful.

This focuses on a change of state:

• first, they were not painful
• then, after walking too much, they started to hurt

By contrast:

• 痛かったです = was painful / hurt

That simply describes the state in the past, without emphasizing the change as much.

So 痛くなりました is very appropriate here because the pain happened as a result of the walking.

It is built like this:

• 痛い = painful
• 痛く = adverbial form of 痛い
• なる = become
• なりました = polite past form of なる

So:

• 痛くなる = to become painful
• 痛くなりました = became painful

This is a very important pattern:

• 寒くなる = become cold
• 眠くなる = become sleepy
• 強くなる = become strong

For i-adjectives, the pattern is:

• い → くなる

少し means a little or slightly, and it modifies the degree of pain.

So:

• 少し痛くなりました = became a little painful
• slightly started to hurt

It comes before the part it modifies, which is very normal in Japanese.

Compare:

• 少し疲れました = I got a little tired
• 少し難しいです = it is a little difficult

Japanese nouns usually do not show singular vs. plural unless it needs to be made clear.

So ひざ can mean:

• knee
• knees

In this sentence, English might naturally translate it as my knees and back, but Japanese leaves that open unless context requires more precision.

The same thing happens with many nouns in Japanese.

Japanese often omits information that is clear from context.

Here, it is obvious that:

• the speaker is talking about their own experience
• the knees and back are their own body parts

So Japanese does not need to say:

• 私は
• 私のひざ
• 私の背中

unless there is a reason to emphasize them.

This is one of the biggest differences from English. Japanese often sounds more natural when it leaves obvious pronouns unstated.

It is mainly a time-topic expression, but は can also give a light contrastive feeling depending on context.

For example, 昨日は can sometimes imply something like:

• as for yesterday...
• yesterday, at least...

In this sentence, though, the main function is simply to set the scene: what happened yesterday.

So a learner should mainly understand it as a natural topic marker here, not worry too much about heavy contrast.

Not in this form.

The correct form here is:

• 歩きすぎて

because it comes from the verb 歩きすぎる, and you are using its て-form.

If you said で, that would not be the normal conjugation of this verb form.

So think of it as:

• 歩きすぎる → 歩きすぎて

just like:

• 食べすぎる → 食べすぎて
• 飲みすぎる → 飲みすぎて

Because it is using polite speech:

• なりました instead of なった

This makes the sentence sound neutral and polite. It is common in conversation with people you are not extremely casual with, in writing for learners, or in general polite narration.

The plain form version would be:

• 昨日は長く歩きすぎて、ひざと背中が少し痛くなった。

Same meaning, but less polite.