Diese Redewendung verstehe ich nicht, deshalb suche ich sie in einem Onlinelexikon.

Redewendung is a feminine noun in German: die Redewendung.

In the sentence, diese Redewendung is the subject, so it is in the nominative case.
Nominative singular forms of dies- (“this”) are:

• dieser – masculine
• diese – feminine
• dieses – neuter

Because Redewendung is feminine and nominative, you need diese Redewendung.

Nicht usually comes:

• before the part of the sentence that is being negated, or
• near the end of the clause if you’re negating the whole statement.

In Diese Redewendung verstehe ich nicht, you’re negating the entire idea “I understand this expression.” There is no second verb or separate phrase after it, so nicht naturally goes towards the end.

Compare:

• Ich verstehe diese Redewendung nicht.
• Diese Redewendung verstehe ich nicht.

Both are correct and mean the same; the difference is emphasis and word order, not the position of nicht relative to the verb phrase.

German main clauses must keep the finite verb in the second position (the V2 rule).

When a sentence starts with deshalb (a sentence adverbial), deshalb occupies the first position. The verb must then come directly after it, and the subject comes after the verb:

• Position 1: deshalb
• Position 2: suche (finite verb)
• Position 3: ich (subject)

So:

• Deshalb suche ich sie … ✅
• Deshalb ich suche sie … ❌ (verb is not in second position)

No. Deshalb is a conjunctive adverb (“therefore / that’s why”), not a subordinating conjunction.

• weil = “because” and introduces a subordinate clause, sending the verb to the end:

  • Ich verstehe diese Redewendung nicht, weil sie schwierig ist.

• deshalb = “therefore / that’s why” and keeps a main clause with normal V2 word order:

  • Ich verstehe diese Redewendung nicht, deshalb suche ich sie …
  • Or as two sentences: … nicht. Deshalb suche ich sie …

So weil gives the reason; deshalb gives the consequence.

The sentence actually contains two main clauses:

1. Diese Redewendung verstehe ich nicht,
2. deshalb suche ich sie in einem Onlinelexikon.

In German, when you join two independent main clauses that are closely related, you may (and usually should) separate them with a comma, even if the connection is only via an adverb like deshalb, trotzdem, dann, etc.

You could also write:

• Diese Redewendung verstehe ich nicht. Deshalb suche ich sie in einem Onlinelexikon.

Both versions are correct; the comma shows the division between the two clauses.

Sie is a personal pronoun referring back to diese Redewendung.

Features:

• Gender: feminine (agreeing with Redewendung)
• Number: singular
• Case: accusative (direct object of suche)

Declension of sie (singular, feminine) is:

• Nominative: sie (she)
• Accusative: sie (her)
• Dative: ihr (to her)

Here, in ich suche sie, it’s accusative: I look for / search for it (the expression).

The formal you is always written with a capital S: Sie, Ihnen, Ihr.

In the sentence we have sie with a lowercase s, so it cannot be the formal “you”. It must be the third-person pronoun:

• lowercase sie = she / they / her / them (depending on context)
• uppercase Sie = you (formal, singular or plural)

Given the context, sie clearly refers back to diese Redewendung (feminine singular).

The preposition in can take either accusative or dative, depending on meaning:

• Accusative (movement into): Ich gehe in den Park.
• Dative (location in): Ich bin im Park.

In ich suche sie in einem Onlinelexikon, you are describing the location where you search (inside/within an online dictionary), not movement into it. So in here takes the dative case.

Dative singular of ein for neuter (das Lexikon) is:

• in einem Onlinelexikon ✅
• in ein Onlinelexikon ❌ (accusative; would suggest movement into it)

Yes, but there is a nuance:

• in einem Onlinelexikon = in an online dictionary (non-specific; any such dictionary)
• im Onlinelexikon = in the online dictionary (specific or assumed to be known)

Im is a contraction of in dem (dative, masculine or neuter). Since Lexikon is neuter (das Lexikon), im Onlinelexikon is grammatically fine, but it refers to one particular dictionary that speaker and listener can identify or assume.

In German, suchen is a very general verb: “to look for / to search (for)”. You can combine it with in to specify the place where you’re searching:

• etwas im Internet suchen – to search for something on the internet
• etwas in einem Buch suchen – to look for something in a book
• etwas in einem Onlinelexikon suchen – to look for something in an online dictionary

Many speakers might actually prefer nachschlagen for “look up (in a reference work)”:

• … deshalb schlage ich sie in einem Onlinelexikon nach.

Your original sentence with suchen is still correct and natural; nachschlagen is just more specific to reference materials.

All three relate to language, but they differ:

• Redewendung – an idiomatic expression / set phrase, often not literally transparent:

  • ins Gras beißen – “to bite the dust” (to die)

• Ausdruck – a wording / expression in a broad sense, not necessarily idiomatic:

  • Dieses Wort ist ein umgangssprachlicher Ausdruck.

• Sprichwort – a proverb, a complete sentence expressing general wisdom:

  • Morgenstund hat Gold im Mund.

In your sentence, Redewendung fits well because it suggests an idiomatic phrase that’s hard to understand from the individual words.

German likes compound nouns: multiple nouns are written together as one word:

• Online
  • Lexikon → Onlinelexikon
• Haus
  • Tür → Haustür
• Wasser
  • Flasche → Wasserflasche

All nouns in German are capitalized, including the last part of a compound and the compound as a whole. Here:

• Lexikon is a noun → capitalized: Lexikon
• Onlinelexikon is a compound noun → capitalized as Onlinelexikon

So Onlinelexikon is correct; writing online Lexikon would look odd to native speakers.

German often uses the present tense to express near future actions when the context makes the time clear:

• Morgen gehe ich ins Kino. – I’m going to the cinema tomorrow.
• Ich rufe dich später an. – I’ll call you later.

In deshalb suche ich sie in einem Onlinelexikon, the action is a direct consequence of not understanding the expression and is either happening now or about to happen. English can choose “I’m looking it up” or “I’ll look it up”; German comfortably stays in the present here.