Bleib bitte im Schatten, solange die Sonne so kräftig scheint.

Bleib is the imperative (command/request) form for du (informal singular) of bleiben:

• infinitive: bleiben
• present (du): du bleibst
• imperative (du): Bleib! In German, the imperative usually drops the subject pronoun (du) unless you want extra emphasis.

Bleib ... is informal (du).
The formal version uses Sie and the infinitive-like imperative:

• Bleiben Sie bitte im Schatten, solange die Sonne so kräftig scheint.

For plural informal (ihr):

• Bleibt bitte im Schatten, solange die Sonne so kräftig scheint.

Bitte is flexible. All of these are common, with slight differences in tone/emphasis:

• Bleib bitte im Schatten ... (very standard)
• Bitte bleib im Schatten ... (a bit more like “Please, stay …”)
• Bleib im Schatten, bitte ... (adds a polite afterthought)

Im is a contraction of in dem:

• in dem Schatten → im Schatten This contraction is extremely common in everyday German.

With in, German chooses the case based on meaning:

• dative = location (where?): im Schatten = “in the shade”
• accusative = direction/movement (where to?): in den Schatten = “into the shade”

Here, the request is to remain located in the shade, so dative fits.

Because solange introduces a subordinate clause (Nebensatz). German generally separates a main clause and a subordinate clause with a comma:

• Bleib bitte im Schatten,
  • subordinate clause
• solange die Sonne so kräftig scheint.

In subordinate clauses introduced by words like solange, German puts the conjugated verb at the end:

• main clause: Bleib ...
• subordinate clause: ... solange die Sonne so kräftig scheint. (verb-final)

That verb-final position is one of the most important word-order rules in German.

• solange = “as long as” (focus on a time span continuing)
• wenn = “if/when” (more general condition or repeated situations)

Here, solange emphasizes: stay in the shade for the entire period that the sun is strong.

So here means “so / so much / that (strong)” and kräftig means “strong, intense.”
Together: so kräftig = “so strong / so intense.”

German often uses so + adjective to refer back to an understood standard (“this strong”).

With Sonne, German naturally uses scheinen to mean “to shine”:

• Die Sonne scheint. = “The sun is shining.”

You could also say:

• solange die Sonne so stark ist (less idiomatic for weather; sounds more like describing a property)
• solange die Sonne so stark scheint (very close in meaning; stark is also common)

Kräftig can mean “strong/powerful” in many contexts (a person, a taste, a color, etc.). For weather/light it means “intense”:

• kräftige Sonne = intense/strong sun
• kräftiger Wind = strong wind
• kräftiger Regen can also mean heavy rain (depending on context)

Grammatically it’s an imperative, but with bitte it’s usually a polite request/advice, often motivated by concern (e.g., avoiding sunburn/heat). Tone depends on context and intonation.

Because they are nouns, and all German nouns are capitalized:

• der Schatten
• die Sonne