Attain means to succeed in achieving something or to reach a specified size or amount. Attain implies an effort to achieve, while obtain is to gain possession of something. However, in everyday usage both attain and obtain are often treated as synonyms.

Assumption means to accept something as true which isn’t founded on strong evidence, while presumption is based on evidence which has some credibility.

Assent indicates the approval of something after careful consideration, while consent implies something is being allowed. For consent, the consenter may not necessarily be happy about the outcome.

Assemblage tends to be an informal collection while assembly refers to a more formalised group, such as a national assembly or school assembly.